I'm not familiar with JFreeChart or the 1.2 components, but I'm just converting some diagram components to JSF 2.0, and they're probably similar.
The components themselves were surprisingly easy to get to work. The main effort was to make sure they generate valid XHTML, and I had a problem with a MethodExpression-valued attribute for which others in this forum gave me a solution. I also had to create a facelets tag file.
I'm guessing that replacing the 1.2 component with a composite component is not applicable since it probably isn't composed of other components, and it is likely to have some chart-specific logic that has to stay in the component.
After that, you can look at making use of resources and doing Ajax. Depending on whether and how it does Ajax, you may not need to do anything.
I found most of my work to be rearranging the demo programs to do Ajax according to the new model.
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