ENH: provide mapbox viewport coordinates in relayout data

I would like the mapbox subplot relayout data to include the lon/lat of the viewport corners. This would make it possible to use a mapbox subplot to display an image layer that is dynamically generated for the current viewport.

The relayout data currently includes mapbox.center, mapbox.zoom, mapbox.bearing, and mapbox.pitch. I would like to propose that we include a mapbox.coordinates property that contains an array of the lon/lat coordinates of the four corners of the viewport.

This would match the coordinates property that is expected in image layers. See https://docs.mapbox.com/mapbox-gl-js/example/image-on-a-map/.

These points can be generated from the mapbox API by following the example in https://github.com/mapbox/mapbox-gl-js/issues/2375#issuecomment-204297638.

Question: Could coordinates be something that is added only to relayout data, or would it need to be a new figure property as well? The reason it’s not clear to me that it should be a property is that it would be read-only. I don’t think we could really support setting coordinates as a way to set where to place the viewport (at least in the case of non-zero bearing and pitch).

[alexcjohnson]

For now though plotly_relayout event data maps exactly to Plotly.relayout arguments, so in principle folks may be using this to accumulate changes that get fed back to the same or another plot later. If we add in other things that look like attributes, this would lead to invalid figures - mostly silently so, but confusing and potentially painful in case there’s any chance we’d add attributes with these names later.

But perhaps we could add something that is clearly NOT an attribute? Like: _derived: {coordinates: ...}

[jonmmease]

Works for me! I’ll open a PR shortly…