Extend multicategory axes to more than 2 levels

This is a request to add nested categorical axes. These are often used in “variability charts”, examples of which can be seen here.

There’s a few ways this could be implemented in practice. Rather than passing an array of values for y the user could pass an object like:

{
  "popcorn": ["gourmet", "gourmet", "plain", ...]
  "batch": ["large", "small", "large", ...]
  ...
}

or simply pass a category name / number (1 or "gourmet-large-little") and pass the nesting information for the axis in the layout section.

(I was surprised not to find an open issue for this – it’s possible that there is one but I didn’t search using the right terms).

Edit: There is some limited support for this as shown here but it involves hacking the axis together by hand and would be difficult to use in an automated manner.

[aaron-kyo]

Can someone please comment on the status of this feature or at least tell us where in the source code we need to look to try and add this capability ourselves?

@etpinard, can you give a high level overview of what we would need to change to extend multicategory axis capability? Any help is greatly appreciated.

Thanks

[richardnm-2]

Ok, just opened a draft PR. This is my first contribution, so please let me know if I have to do anything differently. Also, solved the stacking dividers, calling the remove only on the first pass (last/closest to plot axis). Updated the codepen with this fix with the data from #3723, to show the sorting also working as expected.

I hope this is integrated, please keep me posted on the development and next steps!