\name{xyplot}
\title{Common Bivariate Trellis Plots}
\alias{xyplot}
\alias{xyplot.formula}
\alias{barchart}
\alias{barchart.formula}
\alias{barchart.numeric}
\alias{barchart.default}
\alias{bwplot}
\alias{bwplot.formula}
\alias{bwplot.numeric}
\alias{dotplot}
\alias{dotplot.formula}
\alias{dotplot.numeric}
\alias{dotplot.default}
\alias{stripplot}
\alias{stripplot.formula}
\alias{stripplot.numeric}
\usage{
xyplot(x, data, \dots)
dotplot(x, data, \dots)
barchart(x, data, \dots)
stripplot(x, data, \dots)
bwplot(x, data, \dots)

\method{xyplot}{formula}(x,
       data,
       allow.multiple = is.null(groups) || outer,
       outer = !is.null(groups),
       auto.key = FALSE,
       aspect = "fill",
       panel = "panel.xyplot",
       prepanel = NULL,
       scales = list(),
       strip = TRUE,
       groups = NULL,
       xlab,
       xlim,
       ylab,
       ylim,
       drop.unused.levels = lattice.getOption("drop.unused.levels"),
       \dots,
       default.scales,
       subscripts = !is.null(groups),
       subset = TRUE)


% \method{xyplot}{formula}(x,
%        data,
%        panel = "panel.xyplot",
%        allow.multiple,
%        outer,
%        aspect = "fill", 
%        as.table = FALSE,
%        between,
%        groups,
%        key,
%        auto.key = FALSE,
%        legend,
%        layout,
%        main,
%        page,
%        par.strip.text,
%        prepanel,
%        scales,
%        skip,
%        strip = "strip.default",
%        strip.left = FALSE,
%        sub,
%        xlab,
%        xlim,
%        ylab,
%        ylim,
%        drop.unused.levels,
%        par.settings,
%        perm.cond,
%        index.cond,
%        \dots,
%        default.scales,
% %        panel.groups = "panel.xyplot",
%        subscripts,
%        subset)

\method{dotplot}{formula}(x,
        data,
        panel = "panel.dotplot",
        \dots)

\method{barchart}{formula}(x,
         data,
         panel = "panel.barchart",
         box.ratio = 2,
         \dots)

\method{stripplot}{formula}(x,
          data,
          panel = "panel.stripplot",
          \dots)

% \method{stripplot}{formula}(x,
%           data,
%           panel = "panel.stripplot",
%           jitter.data = FALSE,
%           factor = .5,
%           \dots)

\method{bwplot}{formula}(x,
             data,
             allow.multiple = is.null(groups) || outer,
             outer = FALSE,
             auto.key = FALSE,
             aspect = "fill",
             panel = "panel.bwplot",
             prepanel = NULL,
             scales = list(),
             strip = TRUE,
             groups = NULL,
             xlab,
             xlim,
             ylab,
             ylim,
             box.ratio = 1,
             horizontal = NULL,
             drop.unused.levels = lattice.getOption("drop.unused.levels"),
             \dots,
             default.scales,
             subscripts = !is.null(groups),
             subset = TRUE)



% \method{bwplot}{formula}(x,
%        data,
%        panel = "panel.bwplot",
%        box.ratio = 1,
%        \dots,
%        horizontal,
%        subset = TRUE)
}

\description{
  These are the most commonly used high level Trellis functions to plot
  pairs of variables. By far the most common is \code{xyplot}, designed
  mainly for two continuous variates (though factors can be supplied as
  well, in which case they will simply be coerced to numeric), which
  produces Conditional Scatter plots. The others are useful when one of
  the variates is a factor or a shingle.  Most of these arguments are
  also applicable to other high level functions in the lattice package,
  but are only documented here.
}

\arguments{
  \item{x}{
    The object on which method dispatch is carried out.

    For the \code{"formula"} methods, a formula describing the form of
    conditioning plot. The formula is generally of the form \code{y ~ x
      | g1 * g2 * \dots}, indicating that plots of \code{y} (on the y
    axis) versus \code{x} (on the x axis) should be produced conditional
    on the variables \code{g1, g2, \dots}. However, the conditioning
    variables \code{g1,g2,\dots} may be omitted. The formula can also be
    supplied as \code{y ~ x | g1 + g2 + \dots}.

    For all of these functions, with the exception of \code{xyplot}, a
    formula of the form \code{ ~ x | g1 * g2 * \dots} is also
    allowed. In that case, \code{y} defaults to \code{names(x)} if
    \code{x} is named, and a factor with a single level otherwise.

    Other usage of the form \code{dotplot(x)} is handled by method
    dispatch as appropriate.  The \code{numeric} method is equivalent to
    a call with no left hand side and no conditioning variables in the
    formula.  For \code{barchart} and \code{dotplot}, non-trivial
    methods exist for tables and arrays, documented under
    \code{\link{barchart.table}}.

    The conditioning variables \code{g1, g2, \dots} must be either
    factors or shingles.  Shingles are a way of processing numeric
    variables for use in conditioning. See documentation of
    \code{\link{shingle}} for details. Like factors, they have a
    \code{"levels"} attribute, which is used in producing the
    conditional plots.

    Numeric conditioning variables are converted to shingles by the
    function \code{shingle} (however, using \code{\link{equal.count}}
    might be more appropriate in many cases) and character vectors are
    coerced to factors.

    The formula can involve expressions, e.g. \code{sqrt()},
    \code{log()}.

    A special case is when the left and/or right sides of the formula
    (before the conditioning variables) contain a \sQuote{+} sign, e.g.,
    \code{y1+y2 ~ x | a*b}. This formula would be taken to mean that the
    user wants to plot both \code{y1~x | a*b} and \code{y2~x | a*b}, but
    with the \code{y1~x} and \code{y2~x} superposed in each panel (this
    is slightly more complicated in \code{barchart}). The two parts
    would be distinguished by different graphical parameters. This is
    essentially what the \code{groups} argument would produce, if
    \code{y1} and \code{y2} were concatenated to produce a longer
    vector, with the \code{groups} argument being an indicator of which
    rows come from which variable.  In fact, this is exactly what is
    done internally using the \code{\link{reshape}} function. This
    feature cannot be used in conjunction with the \code{groups}
    argument.  

    To interpret \code{y1 + y2} as a sum, one can either set
    \code{allow.multiple=FALSE} or use \code{I(y1+y2)}.

    A variation on this feature is when the \code{outer} argument is set
    to \code{TRUE} as well as \code{allow.multiple}. In that case, the
    plots are not superposed in each panel, but instead separated into
    different panels (as if a new conditioning variable had been added).
    
    The \code{x} and \code{y} variables should both be numeric in
    \code{xyplot}, and an attempt is made to coerce them if
    not. However, if either is a factor, the levels of that factor are
    used as axis labels. In the other four functions documented here,
    exactly one of \code{x} and \code{y} should be numeric, and the
    other a factor or shingle. Which of these will happen is determined
    by the \code{horizontal} argument --- if \code{horizontal=TRUE},
    then \code{y} will be coerced to be a factor or shingle, otherwise
    \code{x}. The default value of \code{horizontal} is \code{FALSE} if
    \code{x} is a factor or shingle, \code{TRUE} otherwise. (The
    functionality provided by \code{horizontal=FALSE} is not
    S-compatible.)

    Note that this argument used to be called \code{formula} in earlier
    versions (when the high level functions were not generic and the
    formula method was essentially the only method).  This is no longer
    allowed.  It is recommended that this argument not be named in any
    case, but rather be the first (unnamed) argument.
    
    %     All points with at least one of its values missing (NA) in any
    %     of the variates involved are omitted from the plot.
  }
  \item{data}{
    For the \code{formula} method, a data frame containing values for
    any variables in the formula, as well as \code{groups} and
    \code{subset} if applicable.  If not found in \code{data}, or if
    \code{data} is unspecified, the variables are looked for in the
    environment of the formula.  For other methods (where \code{x} is
    not a formula), \code{data} is usually ignored, often with a
    warning.
  }
  \item{allow.multiple, outer}{
    logical flags to control what happens with formulas like \code{y1 +
      y2 ~ x}. See the entry for \code{x} for details.
    \code{allow.multiple} defaults to \code{TRUE} whenever it makes
    sense, and \code{outer} defaults to \code{FALSE} except when
    \code{groups} is explicitly specified or grouping doesn't make sense
    for the default panel function
  }
  \item{box.ratio}{ applicable to \code{bwplot}, \code{barchart} and
    \code{stripplot}, specifies the ratio of the width of the rectangles
    to the inter rectangle space.
  }
  \item{horizontal}{ logical, applicable to \code{bwplot, dotplot,
      barchart} and \code{stripplot}. Determines which of \code{x} and
    \code{y} is to be a factor or shingle (\code{y} if TRUE, \code{x}
    otherwise). Defaults to \code{FALSE} if \code{x} is a factor or
    shingle, \code{TRUE} otherwise. This argument is used to process the  
    arguments to these high level functions, but more importantly, it is
    passed as an argument to the panel function, which is supposed to
    use it as appropriate.

    A potentially useful component of \code{scales} in this case might
    be \code{abbreviate = TRUE}, in which case long labels which would
    usually overlap will be abbreviated. \code{scales} could also
    contain a \code{minlength} argument in this case, which would be
    passed to the \code{abbreviate} function.
  }
  %   \item{jitter.data}{
  %     logical specifying whether the values should be jittered by adding a
  %     random noise in stripplot.  This is actually an argument of
  %     \code{panel.stripplot}.
  %   }
  %   \item{factor}{
  %     numeric controlling amount of jitter as in \code{\link{jitter}}.
  %   }

  The following arguments are common to all the functions documented
  here, as well as most other high level Trellis functions. These are
  not documented elsewhere, except to override the usage given here.

  \item{panel}{
    Once the subset of rows defined by each unique combination of the
    levels of the grouping variables are obtained (see details), the
    corresponding \code{x} and \code{y} variables (or other variables,
    as appropriate, in the case of other high level functions) are
    passed on to be plotted in each panel. The actual plotting is done
    by the function specified by the \code{panel} argument.  Each high
    level function has its own default panel function, which could
    depend on whether the \code{groups} argument was supplied.

    The panel function can be a function object or a character string
    giving the name of a predefined function.

    Much of the power of Trellis Graphics comes from the ability to
    define customized panel functions.  A panel function appropriate
    for the functions described here would usually expect arguments
    named \code{x} and \code{y}, which would be provided by the
    conditioning process.  It can also have other arguments. It might be
    useful to know in this context that all arguments passed to a high
    level Trellis function (such as \code{xyplot}) that are not
    recognized by it are passed through to the panel function. It is
    thus generally good practice when defining panel functions to allow
    a \code{\dots} argument. Such extra arguments typically control
    graphical parameters, but other uses are also common. See
    documentation for individual panel functions for specifics.

    Note that unlike in S-PLUS, it is not guaranteed that panel
    functions will be supplied only numeric vectors for the \code{x} and
    \code{y} arguments; they can be factors as well (but not
    shingles).  Panel functions need to handle this case, which in most
    cases can be done by simply coercing them to numeric.

    Technically speaking, panel functions must be written using Grid
    graphics functions.  However, knowledge of Grid is usually not
    necessary to construct new custom panel functions, there are several
    predefined panel functions which can help; for example,
    \code{panel.grid}, \code{panel.loess}, etc.  There are also some
    grid-compatible replacements of commonly used base R graphics
    functions useful for this purpose.  For example, \code{lines} can be
    replaced by \code{llines} (or equivalently, \code{panel.lines}).
    Note that base R graphics functions like \code{lines} will not work
    in a lattice panel function.

    One case where a bit more is required of the panel function is when
    the \code{groups} argument is not null. In that case, the panel
    function should also accept arguments named \code{groups} and
    \code{subscripts} (see below for details).  A useful panel function
    predefined for use in such cases is \code{panel.superpose}, which
    can be combined with different \code{panel.groups} functions
    determining what is plotted for each group.  See the examples
    section for an interaction plot constructed in this way.  Several
    other panel functions can also handle the \code{groups} argument,
    including the default ones for \code{barchart}, \code{dotplot} and
    \code{stripplot}.

    Even when \code{groups} is not present, the panel function can have
    \code{subscripts} as a formal argument.  In either case, the
    \code{subscripts} argument passed to the panel function are the
    indices of the \code{x} and \code{y} data for that panel in the
    original \code{data}, BEFORE taking into account the effect of
    the \code{subset} argument. Note that \code{groups} remains
    unaffected by any subsetting operations, so
    \code{groups[subscripts]} gives the values of \code{groups} that
    correspond to the data in that panel.  The value of
    \code{subscripts} becomes slightly more complicated when
    \code{allow.multiple} is in effect.  Details can be found in the
    source code of the function \code{latticeParseFormula}.

    A panel function can have two other optional arguments for
    convenience, namely \code{panel.number} and \code{packet.number},
    representing panel order and packet order respectively.  Both
    provide a simple integer index indicating which panel is currently
    being drawn, but differ in how the count is calculated.
    \code{panel.number} is a simple incremental counter that starts with
    1 and is incremented each time a panel is drawn.
    \code{packet.number} on the other hand indexes the combination of
    levels of the conditioning variables that is represented by that
    panel.  The two indices coincide unless the order of conditioning
    variables is permuted and/or the plotting order of levels within one
    or more conditioning variables is altered (using \code{perm.cond}
    and \code{index.cond} respectively), in which case
    \code{packet.number} gives the index corresponding to the
    \sQuote{natural} ordering of that combination of levels of the
    conditioning variables.

    \code{panel.xyplot} has an argument called \code{type} which is
    worth mentioning here because it is quite frequently used (and as
    mentioned above, can be passed to \code{xyplot} directly).  panel
    functions for \code{bwplot} and friends should have an argument
    called \code{horizontal} to account for the cases when \code{x} is
    the factor or shingle.
  }

  %   \item{panel.groups}{
  %     useful mostly for \code{xyplot} and \code{densityplot}. Applies when
  %     \code{panel} is \code{panel.superpose} (which happens by default in
  %     these cases if \code{groups} is non-null)
  %   }

  \item{aspect}{ controls physical aspect ratio of the panels (same for
    all the panels). It can be specified as a ratio (vertical
    size/horizontal size) or as a character string. Legitimate 
    values are \code{"fill"} (the default) which tries to make the
    panels as big as possible to fill the available space; \code{"xy"},
    which \bold{tries} to compute the aspect based on the 45 degree
    banking rule (see \emph{Visualizing Data} by William S. Cleveland
    for details); and \code{"iso"} for isometric scales, where the
    relation between physical distance on the device and distance in the
    data scale are forced to be the same for both axes.

    If a \code{prepanel} function is specified and it returns components
    \code{dx} and \code{dy}, these are used for banking calculations.
    Otherwise, values from the default prepanel function are used.
    Currently, only the default prepanel function for \code{xyplot} can
    be expected to produce sensible banking calculations.  See
    \code{\link{banking}} for details on the implementation of banking .
  }



  \item{groups}{ a variable or expression to be evaluated in the data
    frame specified by \code{data}, expected to act as a grouping
    variable within each panel, typically used to distinguish different
    groups by varying graphical parameters like color and line type.
    Formally, if \code{groups} is specified, then \code{groups} along
    with \code{subscripts} is passed to the panel function, which is
    expected to handle these arguments.  Not all pre-defined panel
    functions know how to, but for high level functions where grouping
    is appropriate, the default panel functions are chosen so that they
    do.

    It is very common to use a key (legend) when a grouping
    variable is specified.  See entries for \code{key}, \code{auto.key}
    and \code{\link{simpleKey}} for how to draw a key.
  }
  \item{auto.key}{
    A logical (indicating whether a key is to be drawn automatically when
    a grouping variable is present in the plot), or a list of parameters
    that would be valid arguments to \code{\link{simpleKey}}. If
    \code{auto.key} is not \code{FALSE}, \code{groups} is non-null and
    there is no \code{key} or \code{legend} argument specified in the
    call, a key is created with \code{simpleKey} with
    \code{levels(groups)} as the first argument. (Note: this may not
    work in all high level functions, but it does work for the ones
    where grouping makes sense with the default panel function)

    \code{simpleKey} uses the trellis settings to determine the
    graphical parameters in the key, so this will be meaningful only if
    the settings are used in the plot as well.

    One disadvantage to using \code{key} (or even \code{simpleKey})
    directly is that the graphical parameters used in the key are
    absolutely determined at the time when the \code{"trellis"} object is
    created. Consequently, if a plot once created is re-\code{print}ed
    with different settings, the parameter settings for the original
    device will be used. However, with \code{auto.key}, the key is
    actually created at printing time, so the key settings will match
    the device settings.
  }
  \item{prepanel}{
    function that takes the same arguments as the \code{panel} function
    and returns a list, possibly containing components named
    \code{xlim}, \code{ylim}, \code{dx} and \code{dy} (and less
    frequently, \code{xat} and \code{yat}).

    The \code{xlim} and \code{ylim} components are similar to the high
    level \code{xlim} and \code{ylim} arguments (i.e., they are usually
    a numeric vector of length 2 defining a range of values, or a
    character vector representing levels of a factor).  If the
    \code{xlim} and \code{ylim} arguments are not explicitly specified
    (possibly as components in \code{scales}), then the actual limits of
    the panels are guaranteed to include the limits returned by the
    prepanel function. This happens globally if the \code{relation}
    component of \code{scales} is \code{"same"}, and on a panel by panel
    basis otherwise. See \code{xlim} to see what forms of the components
    \code{xlim} and \code{ylim} are allowed.

    The \code{dx} and \code{dy} components are used for banking
    computations in case \code{aspect} is specified as \code{"xy"}.  See
    documentation for the function \code{banking} for details regarding
    how this is done.

    The return value of the prepanel function need not have all the
    components named above; in case some are missing, they are replaced
    by the usual component-wise defaults.

    If \code{xlim} or \code{ylim} is a character vector (which is
    appropriate when the corresponding variable is a factor), this
    implicitly indicates that the scale should include the first
    \code{n} integers, where \code{n} is the length of \code{xlim} or
    \code{ylim}, as the case may be.  The elements of the character
    vector are used as the default labels for these \code{n} integers.
    Thus, to make this information consistent between panels, the
    \code{xlim} or \code{ylim} values should represent all the levels of
    the corresponding factor, even if some are not used within that
    particular panel.

    In such cases, an additional component \code{xat} or \code{yat} may
    be returned by the \code{prepanel} function, which should be a
    subset of \code{1:n}, indicating which of the \code{n} values
    (levels) are actually represented in the panel.  This is useful when
    calculating the limits with \code{relation="free"} or
    \code{relation="sliced"} in \code{scales}.

    The prepanel function is responsible for providing a meaningful
    return value when the \code{x}, \code{y} (etc.) variables are
    zero-length vectors.  When nothing is appropriate, values of NA
    should be returned for the \code{xlim} and \code{ylim} components.
  }

  \item{strip}{
    logical flag or function.  If \code{FALSE}, strips are not drawn.
    Otherwise, strips are drawn using the \code{strip} function, which
    defaults to \code{strip.default}.  See documentation of
    \code{strip.default} to see the arguments that are available to the
    strip function.  This description also applies to the
    \code{strip.left} argument (see \code{\dots} below), which can be
    used to draw strips on the left of each panel, which can be useful
    for wide short panels, e.g. in time series plots.
  }

  \item{xlab}{ character string or expression (or a \code{"grob"}) giving
    label for the x-axis.  Defaults to the expression for \code{x} in
    \code{formula}.  Can be specified as \code{NULL} to omit the label
    altogether.  Finer control is possible, as described in the entry
    for \code{main}, with the additional feature that if the
    \code{label} component is omitted from the list, it is replaced by
    the default \code{xlab}.
  }
  \item{ylab}{ character string or expression (or \code{"grob"}) giving
    label for the y-axis.  Defaults to the expression for \code{y} in
    \code{formula}.  Fine control is possible, see entries for
    \code{main} and \code{xlab}.
  }

  \item{scales}{ list determining how the x- and y-axes (tick marks and
    labels) are drawn.  The list contains parameters in
    \code{name=value} form, and may also contain two other lists called
    \code{x} and \code{y} of the same form (described below).
    Components of \code{x} and \code{y} affect the respective axes only,
    while those in \code{scales} affect both.  When parameters are
    specified in both lists, the values in \code{x} or \code{y} are
    used.  Note that certain high-level functions have defaults that are
    specific to a particular axis (e.g., \code{bwplot} has
    \code{alternating=FALSE} for the y-axis only); these can be
    overridden only by an entry in the corresponding component of
    \code{scales}.

    The possible components are :

    \describe{
      \item{\code{relation}}{
	character string that determines how axis limits are calculated
	for each panel.  Possible values are \code{"same"} (default),
	\code{"free"} and \code{"sliced"}.  For \code{relation="same"},
	the same limits, usually large enough to encompass all the data,
	are used for all the panels.  For \code{relation="free"}, limits
	for each panel is determined by just the points in that panel.
	Behavior for \code{relation="sliced"} is similar, except that
	the length (max - min) of the scales are constrained to remain
	the same across panels.

	The determination of what axis limits are suitable for each
	panel can be controlled by the \code{prepanel} function, which
	can be overridden by \code{xlim}, \code{ylim} or
	\code{scales$limits}.  If relation is not \code{"same"}, the
	value of \code{xlim} etc is normally ignored, except when it is
	a list, in which case it is treated as if its components were
	the limit values obtained from the prepanel calculations for
	each panel.
      }
      \item{\code{tick.number}}{
	Suggested number of ticks (ignored for a factor, shingle or
	character vector, in which case there is no natural rule for
	leaving out some of the labels. But see \code{xlim}).
      }
      \item{\code{draw}}{
	logical, defaults to \code{TRUE}, whether to draw the axis at
	all.
      }
      \item{\code{alternating}}{
	logical specifying whether axis labels should alternate from one
	side of the group of panels to the other.  For finer control,
	alternating can be a vector (replicated to be as long as the
	number of rows or columns per page) consisting of the following
	numbers
	\itemize{
	  \item 0: do not draw tick labels
	  \item 1: bottom/left
	  \item 2: top/right
	  \item 3: both.
	}
	\code{alternating} applies only when \code{relation="same"}.
	The default is \code{TRUE}, or equivalently, \code{c(1, 2)}
      }
      \item{\code{limits}}{
	same as xlim and ylim.
      }
      \item{\code{at}}{ location of tick marks along the axis (in native
	coordinates), or a list as long as the number of panels
	describing tick locations for each panel.
      }
      \item{\code{labels}}{
	Labels (strings or expressions) to go along with \code{at}. Can
	be a list like \code{at} as well.
      }
      \item{\code{cex}}{
	numeric multiplier to control character sizes for axis labels.
	Can be a vector of length 2, to control left/bottom and
	right/top separately.
      }
      \item{\code{font}, \code{fontface}, \code{fontfamily}}{
	specifies font for axis labels.
      }
      \item{\code{tck}}{
	numeric to control length of tick marks. Can be a vector of
	length 2, to control left/bottom and right/top separately.
      }
      \item{\code{col}}{
	color of ticks and labels.
      }
      \item{\code{rot}}{
	Angle by which the axis labels are to be rotated. Can be a
	vector of length 2, to control left/bottom and right/top
	separately.
      }
      \item{\code{abbreviate}}{
	logical, whether to abbreviate the labels using
	\code{abbreviate}.  Can be useful for long labels (e.g., in
	factors), especially on the x-axis.
      }
      \item{\code{minlength}}{
	argument passed to \code{abbreviate} if \code{abbreviate=TRUE}.
      }
      \item{\code{log}}{
	whether to use a log scale. Defaults to \code{FALSE}, other
	possible values are any number that works as a base for taking
	logarithm, \code{TRUE}, equivalent to 10, and \code{"e"} (for
	natural logarithm).  Note that in this case the values passed to
	the panel function are already transformed, so all computations
	done inside the panel function will be affected accordingly. For
	example, \code{panel.lmline} will fit a line to the transformed
	values.
      }
      \item{\code{format}}{
	the \code{format} to use for POSIXct variables. See
	\code{\link{strptime}} for description of valid values.
      }
      \item{\code{axs}}{
	character, \code{"r"} or \code{"i"}.  In the latter case, the
	axis limits are calculated as the exact data range, instead of
	being padded on either side. (May not always work as expected.)
      }
    }

    Note that much of the function of \code{scales} is accomplished by
    \code{pscales} in \code{splom}.
  }
  \item{subscripts}{
    logical specifying whether or not a vector named \code{subscripts}
    should be passed to the panel function.  Defaults to \code{FALSE},
    unless \code{groups} is specified, or if the panel function accepts
    an argument named \code{subscripts}. (One should be careful when
    defining the panel function on-the-fly.)
  }
  \item{subset}{
    logical or integer indexing vector (can be specified in terms of
    variables in \code{data}).  Only these rows of \code{data} will be
    used for the plot.  If \code{subscripts} is \code{TRUE}, the
    subscripts will provide indices to the rows of data before the
    subsetting is done.  Whether levels of factors in the data frame
    that are unused after the subsetting will be dropped depends on the
    \code{drop.unused.levels} argument.
  }
  \item{xlim}{ Normally a numeric vector of length 2 (possibly a
    DateTime object) giving minimum and maximum for the x-axis, or, a
    character vector, expected to denote the levels of \code{x}.  The
    latter form is interpreted as a range containing c(1, length(xlim)),
    with the character vector determining labels at tick positions
    \code{1:length(xlim)}

    \code{xlim} could also be a list, with as many components as the
    number of panels (recycled if necessary), with each component as
    described above.  This is meaningful only when
    \code{scales$x$relation} is \code{"free"} or \code{"sliced"}, in
    which case these are treated as if they were the corresponding limit
    components returned by prepanel calculations.

  }
  \item{ylim}{ similar to \code{xlim}, applied to the y-axis. }
  \item{drop.unused.levels}{ 
    logical indicating whether the unused levels of factors will be
    dropped.  Unused levels are usually dropped, but it is sometimes
    appropriate to suppress dropping to preserve a useful layout.  For
    finer control, this argument could also be list containing
    components \code{cond} and \code{data}, both logical, indicating
    desired behavior for conditioning variables and data variables
    respectively.  The default is given by
    \code{lattice.getOption("drop.unused.levels")} , which is initially
    set to \code{TRUE} for both components.
  }
  \item{ default.scales }{
    list giving the default values of \code{scales} for a particular
    high level function.  This should not be of any interest to the
    normal user, but may be helpful when defining other functions that
    act as a wrapper to one of the high level lattice functions.
  }
  \item{\dots}{ further arguments, usually not directly processed by the
    high level functions documented here, but rather passed on to other
    functions. Such arguments can be broadly categorized into two types:
    those that affect all high level Trellis functions in a similar
    manner, and those that are meant for the specific panel function
    used, which may differ across high level functions.

    The first group of arguments are processed by a common, unexported
    function called \code{trellis.skeleton}.  These arguments affect all
    high level functions, but are only documented here, except to
    override the behaviour described here.  All other arguments
    specified in a high level call, specifically those neither described
    here nor in the help page of the relevant high level function, are
    passed unchanged to the panel function used.  By convention, the
    default panel function used for any high level function is named as
    \code{"panel."} followed by the name of the high level function; 
    for example, the default panel function for \code{bwplot} is
    \code{panel.bwplot}.  In practical terms, this means that in
    addition to the help page of the high level function being used, the
    user should also consult the help page of the corresponding panel
    function for arguments that may be specified in the high level call.

    The effect of the first group of common arguments are as follows:

    \describe{


      \item{\code{as.table}:}{ logical that controls the order in which panels
	should be plotted: if \code{FALSE} (the default), panels are drawn
	left to right, bottom to top (as in a graph); if \code{TRUE}, left
	to right, top to bottom.
      }

      \item{\code{between}:}{ a list with components \code{x} and \code{y} (both
	usually 0 by default), numeric vectors specifying the space between
	the panels (units are character heights). \code{x} and \code{y} are
	repeated to account for all panels in a page and any extra
	components are ignored. The result is used for all pages in a
	multi page display. (In other words, it is not possible to use
	different \code{between} values for different pages).
      }

      \item{\code{key}:}{
	A list of arguments that define a legend to be drawn on the plot.
	This list is used as an argument to the \code{\link{draw.key}}
	function, which produces a grid object eventually plotted by the
	print method for \code{"trellis"} objects.


	There is also a less flexible but usually sufficient shortcut
	function \code{\link{simpleKey}} that can generate such a list, as
	well as the argument \code{auto.key} that can be convenient in the
	most common situation where legends are used, namely when there is a
	grouping variable. To use more than one legend, or to have arbitrary
	legends not constrained by the structure imposed by \code{key}, use
	the \code{legend} argument.


	The position of the key can be controlled in either of two possible
	ways. If a component called \code{space} is present, the key is
	positioned outside the plot region, in one of the four sides,
	determined by the value of \code{space}, which can be one of
	\code{"top"}, \code{"bottom"}, \code{"left"} and \code{"right"}.
	Alternatively, the key can be positioned inside the plot region by
	specifying components \code{x}, \code{y} and \code{corner}. \code{x}
	and \code{y} determine the location of the corner of the key given
	by \code{corner}, which can be one of \code{c(0,0)}, \code{c(1,0)},
	\code{c(1,1)} and \code{c(0,1)}, which denote the corners of the
	unit square.  \code{x} and \code{y} must be numbers between 0 and 1,
	giving coordinates with respect to the whole display area.


	The key essentially consists of a number of columns, possibly
	divided into blocks, each containing some rows.  The contents of the
	key are determined by (possibly repeated) components named
	\code{"rectangles"}, \code{"lines"}, \code{"points"} or
	\code{"text"}.  Each of these must be lists with relevant graphical
	parameters (see later) controlling their appearance.  The \code{key}
	list itself can contain graphical parameters, these would be used if
	relevant graphical components are omitted from the other components.


	The length (number of rows) of each such column (except \code{"text"}s)
	is taken to be the largest of the lengths of the graphical
	components, including the ones specified outside (see the entry for
	\code{rep} below for details on this). The \code{"text"} component has to
	have a character or expression vector as its first component, and
	the length of this vector determines the number of rows.

	The graphical components that can be included in \code{key} (and
	also in the components named \code{"text"}, \code{"lines"},
	\code{"points"} and \code{"rectangles"} as appropriate) are:

	\itemize{
	  \item \code{cex=1}
	  \item \code{col="black"}
	  \item \code{lty=1}
	  \item \code{lwd=1}
	  \item \code{font=1}
	  \item \code{fontface}
	  \item \code{fontfamily}
	  \item \code{pch=8}
	  \item \code{adj=0}
	  \item \code{type="l"}
	  \item \code{size=5}
	  \item \code{angle=0}
	  \item \code{density=-1}
	}

	\code{angle} and \code{density} are currently unimplemented.
	\code{size} determines the width of columns of rectangles and
	lines in character widths. \code{type} is relevant for lines;
	\code{"l"} denotes a line, \code{"p"} denotes a point, and
	\code{"b"} and \code{"o"} both denote both together.

	Other possible components of \code{key} are:

	\describe{
	  \item{\code{between}}{
	    numeric vector giving the amount of space (character widths)
	    surrounding each column (split equally on both sides),
	  }
	  \item{\code{title}}{
	    string or expression giving a title for the key
	  }
	  \item{\code{rep}}{
	    logical, defaults to \code{TRUE}.  By default, it's assumed that
	    all columns in the key (except the \code{"text"}s) will have the same
	    number of rows, and all components are replicated to be as long
	    as the longest. This can be suppressed by specifying
	    \code{rep=FALSE}, in which case the length of each column will
	    be determined by components of that column alone.
	  }
	  \item{\code{cex.title}}{
	    cex for the title
	  }
	  \item{\code{lines.title}}{
	    how many lines the title should occupy (in multiples of
	    itself).  Defaults to 2.
	  }
	  \item{\code{padding.text}}{
	    how much space (padding) should be used above and below each
	    row containing text, in multiples of the default, which is
	    currently \code{0.2 * "lines"}.  This padding is in addition to
	    the normal height of any row that contains text, which is the
	    minimum amount necessary to contain all the text entries.
	  }
	  \item{\code{background}}{
	    background color, defaults to default background
	  }
	  \item{\code{border}}{
	    either a color for the border, or a logical.  In the latter
	    case, the border color is black if \code{border} is
	    \code{TRUE}, and no border is drawn if it is \code{FALSE} (the
	    default)
	  }
	  \item{\code{transparent=FALSE}}{
	    logical, whether key area should have a transparent background
	  }
	  \item{\code{columns}}{
	    the number of columns column-blocks the key is to be divided
	    into, which are drawn side by side.
	  }
	  \item{\code{between.columns}}{
	    Space between column blocks, in addition to \code{between}.
	  }
	  \item{\code{divide}}{
	    Number of point symbols to divide each line when \code{type} is
	    \code{"b"} or \code{"o"} in \code{lines}.
	  }
	}
      }
      \item{\code{legend}:}{
	the legend argument can be useful if one wants to place more than
	one key. It also allows one to use arbitrary \code{"grob"}s (grid
	objects) as legends.
	
	If used, \code{legend} must be a list, with an arbitrary number of
	components. Each component must be named one of \code{"left"},
	\code{"right"}, \code{"top"}, \code{"bottom"} or \code{"inside"}.
	The name \code{"inside"} can be repeated, but not the others.  This
	name will be used to determine the location for that component, and
	is similar to the \code{space} component of \code{key}.  If
	\code{key} (or \code{colorkey} for \code{\link{levelplot}} and
	\code{\link{wireframe}}) is specified, their \code{space} component
	must not conflict with the name of any component of \code{legend}.

	Each component of \code{legend} must have a component called
	\code{fun}. This can be a \code{"grob"}, or a function or the name of a
	function that produces a \code{"grob"} when called. If this function
	expects any arguments, they must be supplied as a list in another
	component called \code{args}. For components named \code{"inside"},
	there can be additional components called \code{x}, \code{y} and
	\code{corner}, which work in the same way as it does for \code{key}.
      }

      \item{\code{page}:}{ a function of one argument (page number) to be called
	after drawing each page. The function must be
	\sQuote{grid-compliant}, and is called with the whole display area
	as the default viewport.
      }
      \item{\code{main}:}{ typically a character string or expression or list
	describing the main title to be placed on top of each page. Defaults
	to \code{NULL}.  Can be a character string or expression, or a list
	with components \code{label}, \code{cex}, \code{col} and
	\code{font}.  The \code{label} tag can be omitted if it is the first
	element of the list.  Expressions are treated as specification of
	LaTeX-like markup as in \code{\link{plotmath}}.

	\code{main} can also be an arbitrary \code{"grob"} (grid graphical
	object).
      }
      \item{\code{sub}:}{
	character string or expression (or a \code{"grob"}) for a subtitle
	to be placed at the bottom of each page.  See entry for \code{main}
	for finer control options.
      }
      \item{\code{par.strip.text}:}{ list of  parameters to control the appearance
	of strip text.  Notable components are \code{col}, \code{cex},
	\code{font} and \code{lines}.  The first three control graphical
	parameters while the last is a means of altering the height of the
	strips.  This can be useful, for example, if the strip labels
	(derived from factor levels, say) are double height (i.e., contains
	\code{"\\n"}-s) or if the default height seems too small or too
	large.  The \code{lineheight} component can control the space
	between multiple lines.  Also, the labels can be abbreviated when
	shown by specifying \code{abbreviate = TRUE}, in which case the
	components \code{minlength} and \code{dot} (passed along to the
	\code{\link{abbreviate}} function) can be specified to control the
	details of how this is done.
      }
      \item{\code{layout}:}{
	In general, a Trellis conditioning plot consists of several panels
	arranged in a rectangular array, possibly spanning multiple
	pages. \code{layout} determines this arrangement.

	\code{layout} is a numeric vector giving the number of columns, rows
	and pages in a multi panel display.  By default, the number of
	columns is the number of levels of the first conditioning variable
	and the number of rows is the number of levels of the second
	conditioning variable.  If there is only one conditioning variable,
	the default layout vector is \code{c(0,n)}, where \code{n} is the
	number of levels of the given vector.  Any time the first value in
	the layout vector is 0, the second value is used as the desired
	number of panels per page and the actual layout is computed from
	this, taking into account the aspect ratio of the panels and the
	device dimensions (via \code{par("din")}).  The number of pages is
	by default set to as many as is required to plot all the panels.  In
	general, giving a high value of \code{layout[3]} is not wasteful
	because blank pages are never created.
      }
      \item{\code{skip}:}{
	logical vector (default \code{FALSE}), replicated to be as long as
	the number of panels (spanning all pages).  For elements that are
	\code{TRUE}, the corresponding panel position is skipped; i.e.,
	nothing is plotted in that position.  The panel that was supposed to
	be drawn there is now drawn in the next available panel position,
	and the positions of all the subsequent panels are bumped up
	accordingly.  This is often useful for arranging plots in an
	informative manner.
      }

      \item{\code{strip.left}:}{
	% FIXME: (update: fix what?)
	\code{strip.left} can be used to draw strips on the left of each
	panel, which can be useful for wide short panels, as in time
	series (or similar) plots.  It is a function similar to
	\code{strip}.
      }

      \item{\code{xlab.default}, \code{ylab.default}:}{ fallback default
	for \code{xlab} and \code{ylab} when they are not specified.  If
	\code{NULL}, the defaults are parsed from the Trellis formula.
	This is rarely useful for the end-user, but can be helpful when
	developing new Trellis functions.}

      \item{\code{xscale.components, yscale.components}:}{ 
	functions that determine axis annotation for the x and y axes
	respectively.  See documentation for
	\code{\link{xscale.components.default}}, the default values of
	these arguments, to learn more. }
      
      \item{\code{axis}:}{ function that draws axis annotation.  See
	documentation for \code{\link{axis.default}}, the default value
	of this argument, to learn more. }

      \item{\code{perm.cond}:}{
	numeric vector, a permutation of \code{1:n}, where \code{n} is the
	number of conditioning variables.  By default, the order in which
	panels are drawn depends on the order of the conditioning variables
	specified in the \code{formula}.  \code{perm.cond} can modify this
	order.  If the trellis display is thought of as an
	\code{n}-dimensional array, then during printing, its dimensions are
	permuted using \code{perm.cond} as the \code{perm} argument to
	\code{\link{aperm}}.
      }
      \item{\code{index.cond}:}{
	While \code{perm.cond} permutes the dimensions of the
	multidimensional array of panels, \code{index.cond} can be used to
	subset (or reorder) margins of that array.  \code{index.cond} can be
	a list or a function, with behavior in each case described
	below. 

	The panel display order within each conditioning variable depends on
	the order of their levels.  \code{index.cond} can be used to choose
	a \sQuote{subset} (in the R sense) of these levels, which is then
	used as the display order for that variable.  If \code{index.cond}
	is a list, it has to be as long as the number of conditioning
	variables, and the \code{i}-th component has to be a valid
	indexing vector for the integer vector \code{1:nlevels(g_i)}
	(which can, among other things, repeat some of the levels or
	drop some altogether).  The result of this indexing determines
	the order of panels within that conditioning variable. To keep
	the order of a particular variable unchanged, the corresponding
	component must be set to \code{TRUE}.

	Note that the components of \code{index.cond} are in the order of
	the conditioning variables in the original call, and is not affected
	by \code{perm.cond}.

	Another possibility is to specify \code{index.cond} as a function.
	In this case, this function is called once for each panel,
	potentially with all arguments that are passed to the panel function
	for that panel. (More specifically, if this function has a
	\code{\dots} argument, then all panel arguments are passed,
	otherwise, only named arguments that match are passed.) For a single
	conditioning variable, the levels of that variable are then sorted
	so that these values are in ascending order. For multiple
	conditioning variables, the order for each variable is determined by
	first taking the average over all other conditioning variables.

	Although they can be supplied in high level function calls directly,
	it is more typical to use \code{perm.cond} and \code{index.cond} to
	update an existing \code{"trellis"} object, thus allowing it to be
	displayed in a different arrangement without re-calculating the data
	subsets that go into each panel. In the \code{update} method, both
	can be set to \code{NULL}, which reverts these back to their
	defaults.
      }
      \item{\code{par.settings}:}{
	a list that could be supplied to \code{\link{trellis.par.set}}.
	This enables the user to attach some display settings to the trellis
	object itself rather than change the settings globally.  When the
	object is plotted, these settings are temporarily in effect for
	the duration of the plot, after which the settings revert back
	to whatever they were before.
      }
      \item{\code{plot.args}:}{
	a list of possible arguments to \code{\link{plot.trellis}},
	which will be used by the \code{plot} or \code{print} methods
	when drawing the object, unless overridden explicitly.  This
	enables the user to attach such arguments to the trellis object
	itself.
      }
      \item{\code{lattice.options}:}{
	a list that could be supplied to \code{\link{lattice.options}}.
	This enables the user to attach options settings to the trellis
	object itself rather than change the settings globally.  When
	the object is plotted, these settings are temporarily in effect
	for the duration of the plot, after which the settings revert
	back to whatever they were before.
      }
    }
  }
}
  

\value{

  An object of class \code{"trellis"}. The
  \code{\link[lattice:update.trellis]{update}} method can be used to
  update components of the object and the
  \code{\link[lattice:print.trellis]{print}} method (usually called by
  default) will plot it on an appropriate plotting device.

}
\details{

  All the functions documented here are generic, with the \code{formula}
  method usually doing the actual work.  The structure of the plot that
  is produced is mostly controlled by the formula.  For each unique
  combination of the levels of the conditioning variables \code{g1, g2,
    \dots}, a separate panel is produced using the points \code{(x,y)}
  for the subset of the data (also called packet) defined by that
  combination.  The display can be though of as a 3-dimensional array of
  panels, consisting of one 2-dimensional matrix per page.  The
  dimensions of this array are determined by the \code{layout} argument.
  If there are no conditioning variables, the plot produced consists of
  a single panel.

  The coordinate system used by lattice by default is like a graph,
  with the origin at the bottom left, with axes increasing to left and
  up. In particular, panels are by default drawn starting from the
  bottom left corner, going right and then up; unless \code{as.table =
    TRUE}, in which case panels are drawn from the top left corner,
  going right and then down.  One might wish to set a global preference
  for a table-like arrangement by changing the default to
  \code{as.table=TRUE}; this can be done by setting
  \code{lattice.options(default.args = list(as.table = TRUE))}.  In
  fact, default values can be set in this manner for the following
  arguments: \code{as.table}, \code{aspect}, \code{between},
  \code{page}, \code{main}, \code{sub}, \code{par.strip.text},
  \code{layout}, \code{skip} and \code{strip}.  Note that these global
  defaults are sometimes overridden by individual functions.

  The order of the panels depends on the order in which the conditioning
  variables are specified, with \code{g1} varying fastest. Within a
  conditioning variable, the order depends on the order of the levels
  (which for factors is usually in alphabetical order).  Both of these
  orders can be modified using the \code{index.cond} and
  \code{perm.cond} arguments, possibly using the
  \code{\link[lattice:update.trellis]{update}} (and other related)
  method(s).
}

\note{

  Most of the arguments documented here are also applicable for the
  other high level functions in the lattice package. These are not
  described in any detail elsewhere unless relevant, and this should be
  considered the canonical documentation for such arguments.

  Any arguments passed to these functions and not recognized by them
  will be passed to the panel function. Most predefined panel functions
  have arguments that customize its output. These arguments are
  described only in the help pages for these panel functions, but can
  usually be supplied as arguments to the high level plot.

}
\seealso{
  \code{\link{Lattice}} for an overview of the package, as well as 
  \code{\link{barchart.table}},
  \code{\link{Lattice}},
  \code{\link{print.trellis}},
  \code{\link{shingle}},
  \code{\link{banking}},
  \code{\link{reshape}},
  \code{\link{panel.xyplot}},
  \code{\link{panel.bwplot}},
  \code{\link{panel.barchart}},
  \code{\link{panel.dotplot}},
  \code{\link{panel.stripplot}},
  \code{\link{panel.superpose}},
  \code{\link{panel.loess}},
  \code{\link{panel.linejoin}},
  \code{\link{strip.default}},
  \code{\link{simpleKey}}
  \code{\link{trellis.par.set}}
}
\author{ Deepayan Sarkar \email{Deepayan.Sarkar@R-project.org}}
\examples{

\dontrun{
## wait for user input before each new page (like 'par(ask = TRUE)')
old.prompt <- grid::grid.prompt(TRUE)
}

require(stats)

## Tonga Trench Earthquakes

Depth <- equal.count(quakes$depth, number=8, overlap=.1)
xyplot(lat ~ long | Depth, data = quakes)
update(trellis.last.object(),
       strip = strip.custom(strip.names = TRUE, strip.levels = TRUE),
       par.strip.text = list(cex = 0.75),
       aspect = "iso")

## Examples with data from `Visualizing Data' (Cleveland)
## (obtained from Bill Cleveland's Homepage :
## http://cm.bell-labs.com/cm/ms/departments/sia/wsc/, also
## available at statlib)

EE <- equal.count(ethanol$E, number=9, overlap=1/4)
## Constructing panel functions on the fly; prepanel
xyplot(NOx ~ C | EE, data = ethanol,
       prepanel = function(x, y) prepanel.loess(x, y, span = 1),
       xlab = "Compression Ratio", ylab = "NOx (micrograms/J)",
       panel = function(x, y) {
           panel.grid(h=-1, v= 2)
           panel.xyplot(x, y)
           panel.loess(x,y, span=1)
       },
       aspect = "xy")



## with and without banking

plot <- xyplot(sunspot.year ~ 1700:1988, xlab = "", type = "l",
               scales = list(x = list(alternating = 2)),
               main = "Yearly Sunspots")
print(plot, position = c(0, .3, 1, .9), more = TRUE)
print(update(plot, aspect = "xy", main = "", xlab = "Year"),
      position = c(0, 0, 1, .3))




## Multiple variables in formula for grouped displays

xyplot(Sepal.Length + Sepal.Width ~ Petal.Length + Petal.Width | Species, 
       data = iris, scales = "free", layout = c(2, 2),
       auto.key = list(x = .6, y = .7, corner = c(0, 0)))


## user defined panel functions

states <- data.frame(state.x77,
                     state.name = dimnames(state.x77)[[1]], 
                     state.region = state.region) 
xyplot(Murder ~ Population | state.region, data = states, 
       groups = state.name, 
       panel = function(x, y, subscripts, groups)  
       ltext(x = x, y = y, label = groups[subscripts], cex=1,
             fontfamily = "HersheySans"))

barchart(yield ~ variety | site, data = barley,
         groups = year, layout = c(1,6),
         ylab = "Barley Yield (bushels/acre)",
         scales = list(x = list(abbreviate = TRUE,
                       minlength = 5)))
barchart(yield ~ variety | site, data = barley,
         groups = year, layout = c(1,6), stack = TRUE, 
         auto.key = list(points = FALSE, rectangles = TRUE, space = "right"),
         ylab = "Barley Yield (bushels/acre)",
         scales = list(x = list(rot = 45)))

bwplot(voice.part ~ height, data=singer, xlab="Height (inches)")
dotplot(variety ~ yield | year * site, data=barley)

dotplot(variety ~ yield | site, data = barley, groups = year,
        key = simpleKey(levels(barley$year), space = "right"),
        xlab = "Barley Yield (bushels/acre) ",
        aspect=0.5, layout = c(1,6), ylab=NULL)

stripplot(voice.part ~ jitter(height), data = singer, aspect = 1,
          jitter = TRUE, xlab = "Height (inches)")
## Interaction Plot

xyplot(decrease ~ treatment, OrchardSprays, groups = rowpos,
       type = "a",
       auto.key =
       list(space = "right", points = FALSE, lines = TRUE))

## longer version with no x-ticks

\dontrun{
bwplot(decrease ~ treatment, OrchardSprays, groups = rowpos,
       panel = "panel.superpose",
       panel.groups = "panel.linejoin",
       xlab = "treatment",
       key = list(lines = Rows(trellis.par.get("superpose.line"),
                  c(1:7, 1)), 
                  text = list(lab = as.character(unique(OrchardSprays$rowpos))),
                  columns = 4, title = "Row position"))
}

\dontrun{
grid::grid.prompt(old.prompt)
}


}
\keyword{hplot}
