The graphprune utility.  You are supposed to use this in conjunction with
the archive feature in routers.cgi

This is designed to be called by your favourite batch scheduler - for example, 
cron on a UNIX server.

On UNIX, call it from your crontabs similarly to this:

00 00 * * * /usr/local/bin/graphprune.pl graphprune.pl

You configure it by setting the location of the routers.conf file and your
perl executable in the script, and then set your expiry time for archived
graphs in the routers.conf

Make sure you run it as a user who has rights to delete archived graphs!