Are there any decent free Java data plotting libraries out there?

On a recent Java project, we needed a free Java based real-time data plotting utility. After much searching, we found this tool called the Scientific Graphics Toolkit or SGT from NOAA. It seemed pretty robust, but we found out that it wasn't terribly configurable. Or at least not configurable enough to meet our needs. We ended up digging very deeply into the Java code and reverse engineering the code and changing it all around to make the plot tool look and act the way we wanted it to look and act. Of course, this killed any chance for future upgrades from NOAA.

So what free or cheap Java based data plotting tools or libraries do you use?

Followup: Thanks for the JFreeChart suggestions. I checked out their website and it looks like a very nice data charting and plotting utility. I should have made it clear in my original question that I was looking specifically to plot real-time data. I corrected my question above to make that point clear. It appears that JFreeChart support for live data is marginal at best, though. Any other suggestions out there?

I've had success using JFreeChart on multiple projects. It is very configurable. JFreeChart is open source, but they charge for the developer guide. If you're doing something simple, the sample code is probably good enough. Otherwise, $50 for the developer guide is a pretty good bargain.

With respect to "real-time" data, I've also used JFreeChart for these sorts of applications. Unfortunately, I had to create some custom data models with appropriate synchronization mechanisms to avoid race conditions. However, it wasn't terribly difficult and JFreeChart would still be my first choice. However, as the FAQ suggests, JFreeChart might not give you the best performance if that is a big concern.

I just ran into a similar issue (displaying fast-updating data for engineering purposes), and I'm using JChart2D. It's pretty minimalist and has a few quirks but it seems fairly fast: I'm running a benchmark speed test where it's adding 2331 points per second (333x7 traces) to a strip chart and uses 1% of the CPU on my 3GHz Pentium 4.