add map legend & align # of color bins

The current maps don't contain legends. Would be great to add those.
Also, the # of bins in the color bar don't yet get adjusted to match the # of bins in a variable, e.g. in this example of a binary variable where the color bins include more than 2:

No files or tests are named. Start by locating the map rendering entry point and the logic that builds the color bar; compare its bin count with binary and other variables. Done means maps display legends and the color bar uses the variable's actual number of bins.

Written by the indexing model from the issue text.