The Class Diagram Pane draws a diagram for the class hierarchy surrounding the currently selected class. This will show the classes in the superclass chain and all of the subclass tree. If you click over a class in the diagram this will become the selected class in the browser.

You can expand and collapse the nodes in the tree using the [+] and [-] buttons next to each class icon.

The Class Diagram Pane is a class browser plug-in and is implemented by the ClassDiagramPlugIn class in the image.