Feature request: categorical heatmap

[Batalex]

Hi,

it would be quite handy to have a categorical mode for heatmaps.
As of today heatmaps are rendered with a gradient colorbar (which can be hidden).

The closest thing to a categorical heatmap with Plotly is the annotated heatmap figure factory periodic table example, but it is still a bit tedious to use since:

• we need to specify the colorscale by hand (and it needs to match the number of different values or the wrong colors are picked)
• we need to hardcode the legend directly in the trace.

[etpinard]

Thanks for writing in!

Your request will be part of bigger project, tracked in #1747