jfreechart does not provide basic OSGi metadata and can hence not be used in an OSGi context. I've attached a patch for current trunk which adds ant tasks to build.xml which create such OSGi-ified jars:
$ cd path/to/trunk
$ patch < generate_osgi_metadata.patch
$ ant make-jfreechart-bundle
That produces a jar in the base dir with an -osgi. suffix. This file can be used in an OSGi context as well as it provides basic export-package statements.
I've also created a pull request on github for jfreechart-fse which does basically the same thing using maven.
Please consider including this patch for your next release and consider shipping jfreechart as bundle instead of jar. Let me know if you have questions.