another caveat (for what it's worth): As it happens I, too, just had a
(short) look at Processing (for Java projects unrelated to MASON), and the
following statement made me cautious:

"The main rule when using Java code: You cannot use most of the AWT or Swing
(which is built on the AWT), because it will interfere with the graphics
model. If you want to add scroll bars and buttons to your projects, you
should make them using Processing code, or embed your Processing applet
inside another Swing or AWT application (see below). Even if they appear to
work, such sketches will usually break when you try to run on other
operating systems or other versions of Java. "
(from the Processing FAQ, question 
"I know Java, is this Java? How do I use it that way?"

My reading of this would be that apparently one *can* integrate Processing
into one's own Java projects in some way, but any GUI customization outside
of Processing then seems to be tricky or impossible. 

And as has been said already, Processing certainly has a scope and focus
different from JFreeChart: "to create images, animations, and interactions"
rather than generating diagrams for data evaluation.