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.

Comienza revisando la compatibilidad limitada existente con los ejes de subcategorías descrita en la documentación enlazada de ejes de Plotly y en los ejemplos de gráficos de variabilidad. Aclara si la API debería aceptar arrays de categorías anidados o metadatos de anidamiento a nivel de layout y, después, sigue el comportamiento actual del eje multicategory. Se considera terminado cuando los ejes categóricos anidados funcionan automáticamente sin construir los ejes manualmente.