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.

Commencez par examiner la prise en charge limitée existante des axes de sous-catégories décrite dans la documentation liée sur les axes Plotly et dans les exemples de graphiques de variabilité. Déterminez si l’API doit accepter des tableaux de catégories imbriqués ou des métadonnées d’imbrication au niveau du layout, puis retracez le comportement actuel de l’axe multicategory. Le travail est terminé lorsque les axes catégoriels imbriqués fonctionnent automatiquement sans construction manuelle des axes.