\name{identify}
\alias{identify}
\alias{identify.default}
\title{Identify Points in a Scatter Plot}
\usage{
identify(x, \dots)

\method{identify}{default}(x, y = NULL, labels = seq(along = x), pos = FALSE,
         n = length(x), plot = TRUE, atpen = FALSE, offset = 0.5,
         tolerance = 0.25, \dots)
}
\arguments{
  \item{x,y}{coordinates of points in a scatter plot.  Alternatively, any
    object which defines coordinates (a plotting structure, time
    series etc: see \code{\link{xy.coords}}) can be given as \code{x},
    and \code{y} left undefined.}
  \item{labels}{an optional character vector, the same length as \code{x} and
    \code{y}, giving labels for the points.  Will be coerced using
    \code{\link{as.character}}.}
  \item{pos}{if \code{pos} is \code{TRUE}, a component is added to the
    return value which indicates where text was plotted relative to each
    identified point: see Value.}
  \item{n}{the maximum number of points to be identified.}
  \item{plot}{logical: if \code{plot} is \code{TRUE}, the labels are printed at
    the points and if \code{FALSE} they are omitted.}
  \item{atpen}{logical: if \code{TRUE} and \code{plot = TRUE}, the
    lower-left corners of the labels are plotted at the points clicked
    rather than relative to the points.}
  \item{offset}{the distance (in character widths) which separates the
    label from identified points.  Ignored if \code{atpen = TRUE} or
    \code{pos = 0}.}
  \item{tolerance}{the maximal distance (in inches) for the pointer to be
    \sQuote{close enough} to a point.}
  \item{\dots}{further arguments passed to \code{\link{par}} such as
    \code{cex}, \code{col} and \code{font}.}
}
\description{
  \code{identify} reads the position of the graphics pointer when the
  (first) mouse button is pressed.  It then searches the coordinates
  given in \code{x} and \code{y} for the point closest to the pointer.
  If this point is close enough to the pointer, its index will be returned as
  part of the value of the call.
}
\details{
  \code{identify} is only supported on screen devices such as
  \code{X11}, \code{windows} and \code{quartz}.  On other devices the
  call will do nothing.
  
  If \code{plot} is \code{TRUE}, the point is labelled with
  the corresponding element of \code{text}.
  The labels are placed below, to the left, above or to the right
  of the identified point, depending on where the cursor was relative to
  the point.

#ifdef unix
  For the usual \code{\link{X11}} device the identification process is
  terminated by pressing any mouse button other than the first.
  For the \code{\link{quartz}} device the process is terminated by
  pressing the \code{ESC} key.
#endif
#ifdef windows
  The identification process is terminated by clicking the second button
  and selecting \sQuote{Stop} from the menu, or from the \sQuote{Stop}
  menu on the graphics window.
#endif
 
  On most devices which support \code{identify}, successful selection of
  a point is indicated by a bell sound unless
  \code{\link{options}(locatorBell = FALSE)} has been set.

  If the window is resized or hidden and then exposed before the identification
  process has terminated, any labels drawn by \code{identify}
  will disappear.  These will reappear once the identification process has 
  terminated and the window is resized or hidden and exposed again.
  This is because the labels drawn by \code{identify} are not
  recorded in the device's display list until the identification process has
  terminated.
}
\value{
  If \code{pos} is \code{FALSE}, an integer
  vector containing the indexes of the identified points.

  If \code{pos} is \code{TRUE}, a list containing a component
  \code{ind}, indicating which points were identified and a component
  \code{pos}, indicating where the labels were placed relative to the
  identified points (1=below, 2=left, 3=above, 4=right and 0=no offset,
  used if \code{atpen = TRUE}).
}
\references{
  Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
  \emph{The New S Language}.
  Wadsworth \& Brooks/Cole.
}
\seealso{
  \code{\link{locator}}
}
\keyword{iplot}