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.

首先查看链接的 Plotly 轴文档和变异性图表示例中所描述的现有限制性子类别轴支持。明确 API 应接受嵌套类别数组，还是接受布局级嵌套元数据，然后追踪当前 multicategory 轴的行为。当嵌套分类轴无需手动构建即可自动工作时，即视为完成。