During my university times, I often used EvalDraw as quick 2D/3D graphing calculator:

http://advsys.net/ken/download.htm

You type in some C-ish code, it gets immediately executed and drawn. Instant feedback was a winner feature here. Graphing y=f(x) was AFAIR as simple as typing "(x)" in the first line and writing an expression in the next. "(x,t)" for time-varying singals, "(x,y)" for 2-d functions with values viewed as colors, "(x,y,z,&r,&g,&b)" for full-voxels, etc.

I don't even remember how this little program found its way to my hard drive, but I had a lot of good times with it.