Split the display in two with a pane, we'll use the bottom half to show information about the current revision. Add a Back and Forward button which figure out which revision is logically the next of previous and moves the cursor to it. Handle the cursor-changed event to enable/disable the buttons (and soon update the bottom pane).
Further split up graph.py so we can stash the internal lists to do the above; also it may allow us in future to produce partial graphs.