Note to self: RRDtool
We use gnuplot at work to make our custom one-off pretty graphs that monitor traffic:
It's kind of pain to use, I think largely because of it's history as an X11/postscript graphing tool. If you just want a PNG, it's not really the sweet spot. Gnuplot seems better suited to plotting equations than it does to reams of traffic data.
I noticed our Google mini uses rrdtool; it looked a lot like gnuplot, so I'm looking into it further to see if it's easier to do little consoles like ours.
Update: needs freetype2 and libpng:
yum install libpng-devel
yum install freetype-devel