This problem toss me for 2 or 3 days. Go to the JFreeChart official forum and also searched, and some people have put forward the same problem, and they did not find the answer.
Later, I saw someone told that the Class file was packed into a package, tried it, and solved the problem.
Old:
The front is required for JFreeChart run, and then one is a tool class that bugapplet1 runs.
See you detailed:
http://java.sun.com/docs/books/tutorial/applet/appletsonly/html.html
But when running, the ClassNotFoundExcetPion is thrown, and the applet is an error when loading the multi-file load.
Pack JFreeChart-0.9.20.jar, JCOMMON-0.9.5.jar and Util.jar are packaged into a JAR file so that there will be no problem.
