Math graphing program for Linux

[Chuck Noyes]

Does anyone know of a program that will handle 2D or 3D mathematical
graphing, where the user would specify some function y=f(x) and the program
would plot the
equation on an xy or xyz graph?

I know of a $100+ package called *Mathematica*, but that might be a little
over-kill for what I'm looking for.  This would be for a student not a
professional engineer.

Thanks,   Chuck