Scale sorting

[kalealex]

Ziyang did and amazing job adding the sorting indices to guarantee that ordinal variables (e.g., month, day) always appear in a consistent order. We just need a couple other fixes related to this functionality.

Todo

• Please reverse the default sort order for factors on the y-axis. It's correct for model checks, but without model checks the tool defaults to the opposite sort order. This means that adding model checks flips the order of the y-axis for ordinal variables, which could be jarring for users.
• There should be a sort index for modelcheck_group so that model groups are mapped to colors in the order they are applied by the user: ["data", , , ...]. This can be derived from the models object in the state of the tool.

There's also the question of whether each model should get a consistent color from the time the user adds it to the time they remove it. The difficulty with this is that if I add three models and remove the first one, the sort index approach would reassign the colors of the second and third models added. I'm not sure this is a big enough deal to fuss with custom color scales in Vega.

[kalealex]

The code we have to do this still leads to undesired program behavior. We'll have to come back to this when we have time to fix it.