Feature/scatterplot interactivity

This doesn't add legends yet, but otherwise allows for four different scatterplot dimensions to be switched with dropdowns:  both axes, size, and color.

The map color service got refactored out so that the chart could share it.
I'm in the habit of writing it that way to indicate that it's pointing to zero, as a hint.  Another developer started me doing that.  I think that syntax may exist in the map service, too.