Forgot to mention I'm using Seam 2.0.1.GA
I had the same problem. I was using jfreechart 1.0.9. I solve the issue with another release : 1.0.8a
the problem actually went away for me when I upgraded Seam.
Pointing out the jfreechart version to use for others deserves some rating points, I had to dig around in poms to find that (or did I check the MANIFEST.MF of the bundled jars, can't remember) ;)