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.

まず、リンク先の Plotly の軸に関するドキュメントと変動性チャートの例で説明されている、既存の限定的なサブカテゴリ軸のサポートを確認します。API がネストされたカテゴリ配列を受け入れるべきか、それともレイアウトレベルのネストメタデータを受け入れるべきかを明確にし、その後、現在の multicategory 軸の動作を追跡します。手動で軸を構築しなくても、ネストされたカテゴリ軸が自動的に機能すれば完了です。