Plplot is on sourceforge: http://sourceforge.net/project/showfiles.php?group_id=2915
PLplot is relatively small, portable, freely distributable, and is rich enough to satisfy most users. It has a wide range of plot types including line (linear, log), contour, 3D, fill, and almost 1000 characters (including Greek and mathematical) in its extended font set. The package is designed to make it easy to quickly get graphical output; only a handful of function calls is typically required. For more advanced use, virtually all aspects of plotting are configurable.
A binary compile of an extension for WinTk is available here: ftp://ftp.ucsd.edu/pub/alpha/tcl/extensions/plplot5.1a.zip Also see http://www.santafe.edu/~vince/Pltk.html for some screenshots.
The cvs distribution contains many examples (in examples/tcl and examples/tk) which work well with this binary release.
Plplot currently uses 'fopen' etc directly so will not work properly at accessing its library files from inside a scripted document.
Vince was responsible for making Plplot work with Tk cross-platform.