In the process, I also changed a bit the visualization:
- deal with the error caused by classes that do not have a superclass
- make better use of space: number of classes is now mapped on the
color and the tree layout has a larger vertical gap
Still, the edges should be dimmer because they are typically many of
them (so you want them to be in the background), but I did not have
enough time to dig into how the colors are assigned.