JFreeChart library, recommended!
I'm finishing a small project using JFreeChart and this (LGPL) thing rocks!
The library structure is very well thought out, with the nice "it's easier than you thought" feeling that good design provides. Still, it stays flexible, either by using the extensive graph configuration features or by plugging custom renderers for more complex needs.
Performance is so-so when generating bitmaps, but I haven't investigated much as it is adequate for this project.
Such a library is a perfect example of the power of Open Source: I can't imagine finding my way in it without browsing the source code directly in my IDE, or it would have taken me much longer.
Definitely recommended - and remember, Fins lets you integrate JFreeChart with Cocoon!