Handle consistent colors in charts when factor levels are missing

[sfirke]

In particular, for our most common scales (e.g., "Strongly agree" onward).

This took some work to get functional:

Note the correct colors in the middle bars, as if the others were there.

I leaned on this answer: https://stackoverflow.com/a/22822231 but it would be nice to not have to hack this together each time.