The plotting functionality makes use of the JFreeChart library, a popular open source Java framework for creating diagrams and charts. Another example of one of the benefits of the marriage of Java and Python that is Jython.
It reminds me of the run of such posts I went on this past spring. JFreeChart might be great, but Java platform developers shouldn't underestimate what matplotlib brings to the C Python platform: loads of tools for earth science visualization and excellent integration with tried and true numerical libraries. That said, I'll freely admit to some JTS envy.
BTW, Shapely 1.2.1 is now accepted to Debian unstable.