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).

Empieza leyendo el manejo de hovertemplate y cómo se resuelven las rutas de pmeta para los mapas de calor. Determina cómo deben evaluarse los elementos de ruta basados en x e y y, después, verifica que el texto del hover pueda usar las formas solicitadas %{data.pmeta.foo.%{x}} y %{data.pmeta.bar.%{y}} sin requerir texto por entrada.