Annotated Heatmap: automatic font color

Given an annotated heatmap I would expect the cell text to contrast with the cell color.
(Not doing so leads to the readability issues in the current documentation/example of Annotated Heatmap: https://plotly.net/simple-charts/heatmaps.html)

A good contrast is shown and discussed on the python side of plotly: https://github.com/plotly/plotly.py/issues/1300

When I did not find a way to do that, I looked at the implementation of AnnotatedHeatmap.
Here the configuration of the annotations is not dependent on the actual cell values - I could only apply a workaround of setting a background color.
I did not find/check the actual layouting step if there is a better place to compute correct font color.

Inizia leggendo l’implementazione di AnnotatedHeatmap e la documentazione o l’esempio di annotated heatmap collegati nell’issue, quindi confronta il comportamento del contrasto discusso in plotly.py issue 1300. Determina dove il colore del carattere delle annotazioni può tenere conto dei valori delle celle e termina quando il testo annotato rimane leggibile su tutti i colori della heatmap.