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.

Start by reviewing the existing limited subcategory-axis support described in the linked Plotly axes documentation and the variability-chart examples. Clarify whether the API should accept nested category arrays or layout-level nesting metadata, then trace the current multicategory axis behavior. Done means nested categorical axes work automatically without manual axis construction.