Sticky colors for nominal features in Facets Dive

[benhutch]

When coloring examples by color, it would be useful to have colors be "sticky" and consistently map to the same feature values.

I have some closely related data sets which use the same features however the frequency of the feature values differs across data sets. The coloring currently seems to be based on the frequency of the feature values, so the colors in my data sets do not match.