The cewolf example web app shows a couple of combined charts under "Complex Charts". You can study its JSP files to help get you going. If that doesn't help, post what you have so far, and let us know where you have problems progressing.
The web app I pointed to above has a number of examples of how to create DatasetProducers. The mechanics vary a bit depending on what kind of chart you want to create, but it shouldn't be too hard to adapt one of the examples.
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