Gnuplot is awesome. I've been hyping it up for years [1]. It's simple to get up and running with it, and remarkably deep when needed. Plays really well with poor man's command line data analytics (e.g. cut and awk).
Thanks for the link! One of the quite astounding things about gnuplot is that it continues to make steady progress, decade after decade, not stagnating or flaming out, without massive attention or lots of institutional support, and with great backwards compatibility. From the outside at least, it seems to be a very well run project, with admirable long term focus.