The usual causes of classpath problems are (in order of frequency):

  • New users that don't really understand what the classpath is about, and how it works. If this is you, find a good introductory book on Java and read up about the classpath until you understand it. You won't get anywhere with Java until you understand how Java finds and loads classes.
  • You have more than one copy of the JFreeChart and/or JCommon jar files on the current classpath.
  • You have an out-of-date version of JCommon (this is not really a classpath issue, but users often report it as such).
  • Very occasionally, a jar file can be corrupted for whatever reason. If nothing else works for you, try downloading a fresh copy of the required jar file(s) (this goes for any jar file, not just the JFreeChart/JCommon jar files).

Source: JFreeChart FAQ