I spent a little time installing the GD graphics library on my server and then scouted out some PHP graphing libraries. I ended up with PHPGraphLib which is about as stripped down as you could imagine. The results were underwhelming.
There's another one called pChart that seems a lot more capable, but I tried to use it and mistook m own incompetence installing GD for its failings, so I punted on it and went to PHPGraphLib. It turns out that all I needed to do was reboot the server* and I was in business. I'm going to re-install the pChart library and try that. This one looks like some researcher needed some quick and dirty graphing functions and threw this one together. It hasn't been updated in quite some time.
* - Rebooting. Is there no problem it can't solve?