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.

首先檢視連結的 Plotly 軸文件與變異性圖表示例中所描述的現有限制性子類別軸支援。釐清 API 應接受巢狀類別陣列，還是接受版面配置層級的巢狀中繼資料，然後追蹤目前 multicategory 軸的行為。當巢狀分類軸無需手動建構即可自動運作時，即視為完成。