I'd recommend, of course, the JFreeChart Developer Guide:
http://www.object-refinery.com/jfreechart/guide.html
As well as the guide, all the source code for the demo applications is included for download with the JFreeChart Developer Guide. Sales from the guide keep the project running, so please try to lend your support!
There are some free options, a couple of articles are linked on the JFreeChart home page and there are a few simple demos in the org.jfree.chart.demo.* package. And you'll find a lot of useful free information in this forum.