Implement Legends for HeatMap

[arjxn-py]

Use case

Legends currently works for simple, graduated & categorised symbology but not for heatmaps. #880

Bonus question: Can we make legends work for canonical symbology too?

Preferred solution

No response

Alternative solutions

No response

[mfisher87]

Bonus question: Can we make legends work for canonical symbology too?

I can't think of what this implementation might look like. We don't really have enough information about how the data provider settled on the colors they put in the data. They could be completely uncorrelated with any variables in the data.