Add legend for data-driven layers from Mapbox Studio

[otravers]

In Mapbox Studio one can assign properties such as Fill Color based on data conditions:

It would be nice to be able to surface these up as a optional tileset legend (separate from the PBI data legend), where the author would select whether to feature any of the tileset's data-driven criteria (e.g. building color or pattern by type, traffic condition colors).

[ryanbaumann]

Agree, this would be a powerful feature - thanks for the request @otravers . It hits on a larger category of feature requests involving "using map layers as data sources" in Power BI.

cc/ @samgehret

[otravers]

"Using map layers as data sources in Power BI": YES! (Related: #72)

For further illustration purposes, and in an unsuccessful attempt to crash Mapbox/PBI, here's a map with:

1. Data-driven choropleth done in Mapbox Studio, since Mapbox in Power BI won't combine the Choropleth data with other data series. (This comment should earn me passive aggressive points.)

2. Data-driven, color-coded Points of Interest. Speaking of which, I wish I could similarly color the POI icons [Edit: see https://github.com/Support icon-color for non-SDF icons mapbox-gl-js#3605].

3. Data-driven, color-coded 3D buildings. Incidentally the underlying data seems way less qualified than POIs, but I guess that's OpenStreetMap's problem.

4. Color-coded traffic congestion.

5. Satellite + streets background layers, because at this point, why not?

6. Power BI data layer with Circles, which is the only one that can be explained to the end user via a legend.