Color by category within one dimension, for parcats (and parcoords?)

[alexcjohnson]

In #2963, when @jonmmease introduced the parcats trace type, he showed some existing examples like this one:

One thing this and other examples he showed do that parcats does not is give a categorical colorscale to paths based on one of their dimensions. We let users specify per-path color only with a new line.color array, which is nice for programmatic applications like brushing but not as useful for simply exploring within one parcats diagram. In fact, all these examples color by the first dimension, but in principle it would be nice to be able to color by any dimension. Users should be able to use either layout.colorway or a different colorway, and we should be careful to keep the colors from changing if you drag categories around. I could also imagine letting users perhaps doubleclick a category to switch the coloring to that category.

parcoords might like this feature as well; in that context it would be a colorscale related to a specific dimension.

[jonmmease]

I think this would work out pretty naturally once we have categorical color scales (#1747).

Basically setting parcats.line.color to the parcats.dimension.values array of any dimension would accomplish the coloring. Then the doulbeclick support could switch out which dimension's values are used for coloring.

[alexcjohnson]

Thinking about this in the context of uirevision #3236, while a values array should definitely be supported, using a reference to the dimension would be preferable when that's what you're doing. That way if the underlying data is updated the colors follow along.

[gvwilson]

Hi - this issue has been sitting for a while, so as part of our effort to tidy up our public repositories I'm going to close it. If it's still a concern, we'd be grateful if you could open a new issue (with a short reproducible example if appropriate) so that we can add it to our stack. Cheers - @gvwilson