PyViz rendering of CSMA links

When a CSMA link with more than two nodes is within a topology, PyViz fails to render it, and reports a number of drawing failures, such as:

drawing failure for widget 'GtkVBox': invalid matrix (not invertible)

For example:

$ ./waf --run udp-echo --vis

fails in this way. We are looking for a patch to ns-3-dev that would allow such programs to run successfully in PyViz.