Arcs linking nodes in trees

[JMCanning78]

The current Trees.py visualization draws the arcs linking the nodes in the binary tree as starting and ending at the bottom or top of the node's circle. Here's a sample.

It would be preferable to have the arcs end at the centers of the nodes and be hidden behind the node's circle like this:

The node circles would need a fill color that is opaque so they obscure the line segments.