Allow indexing data in pmeta using x and y coordinates

Problem Description.

Displaying a heatmap, I'd like to include in the hover box some data which depends only on the X (column) or Y (row).

Describe the solution you'd like

Allow specifying %{data.pmeta.foo.%{x}} and/or %{data.pmeta.bar.%{y}} in the hovertemplate.
In general, if a path element is in the format %{...}, then evaluate it, and use this as the next key.

Describe alternatives you've considered

It is possible to specify per-entry hover text for a heatmap, but this would require precomputing and sending much more data between the client and the server (N * M complete texts instead of N + M text fragments).

首先阅读 hovertemplate 的处理方式，以及热力图中 pmeta 路径的解析方式。确定应如何计算基于 x 和 y 的路径元素，然后验证悬停文本是否可以使用所请求的形式 %{data.pmeta.foo.%{x}} 和 %{data.pmeta.bar.%{y}}，而不需要为每个条目提供文本。