Add color key/legend

[benloh]

This implements a simple legend display across the bottom of the graph.

NOTES:

• The bottom of the screen seemed like the ideal place to put a legend because it doesn't take up much room and would cover less of the graph.
• The downside is that if you have a LOT of types or long labels, we would run out of room.
• The keys can wrap, though more than one line tends to get cut off by the accessible message.