Improved graph graphics

[zyzek]

Desiderata:

• A legend for graph colours;
• Less obstructive side panel;
• Better interactivity, (zooming/panning, search, improved display format for stacks etc., links between def and use sites);
• Better style

Something like Plotly or Bokeh may be handy here if the thing is to be interactive, but perhaps it's just better to have a JS viewer which just reads a json dump of the CFG.