Here is something to keep an eye on

http://www.ejschart.com/index.php

It's a dam impressive javascript charting library. I couldn't find any references to how it was going to be licensed however. If it's going to be open sourced under the LGPL, MPL or BSD/MIT licences then we will be able to integrate it with SilverStripe.

that is definitely the most advanced/impressive js charting library ive come across, but i havent seen any indication of it being open sources
i could be missing that though

seeing as ss already uses prototype though, plotr is a pretty nice library
not nearly as comprehensive as ejachart, but not bad
http://www.solutoire.com/plotr