Allow setting zmid with zmin and zmax in plotly.graph_objects.Heatmap

Feature request

I've been trying to plot a subset of a ranked list (gene differential expression dataset). I would like the minimum and maximum color scale set to that of the original complete ranked list, with the midpoint set to zero. It currently seems to be impossible in plotly to do this. I can plot the subset with the colorscale min and max set to the original dataset min and max, but this causes the midpoint to no longer be set to zero.

It can be desirable to set a colorscale to the minimum and maximum of a positive/negative spanned dateset with unequal absolutes but to still have the midpoint at zero without needing to construct and entirely custom colorscale. zmid being ignored when zmax and zmin are set just seems a super odd behavior to me.

Inizia da plotly.graph_objects.Heatmap e segui come vengono gestiti zmin, zmax e zmid per la colorscale. Controlla i test esistenti di Heatmap per verificare il comportamento correlato. Il lavoro è completato quando zmid rimane effettivo anche se zmin e zmax sono impostati esplicitamente, con una copertura per limiti positivi e negativi disuguali.