gnuplot is a portable command-line driven interactive data and function plotting utility for UNIX, IBM OS/2, MS Windows, DOS, Macintosh, VMS, Atari and many other platforms. See http://www.gnuplot.info/
Warning: The following site appears to be a site for gnuplot but is not:
Some wikis support the gnuplot functionality directly inside the wiki context; i.e. TWiki has http://twiki.org/cgi-bin/view/Plugins/GnuPlotPlugin for this purpose.