Support legends

Created by: slifty

We need to be able to support the addition of a legend component such as the one shown in the choropleth demo

This can be done in a few ways: it could just take generic HTML or it could take a more structured list of items...

Immediately I can think of two kinds of legend:

1. Color gradient legend (e.g. the choropleth example).
2. Icon legend (e.g. showing icons images and indicating which is which).

Are there other types of map legend?

It seems likely we should do the "take structured data and generate the legend" over the "take HTML and just render it" approach.