[FEATURE]: Heatmap with custom colour for NaN values

Description

Currently, z-values of NaN get rendered as transparent in heatmaps. Adding a new parameter nancolor to the heatmap would fix this by allowing users to set a specific colour for NaN values.

Why should this feature be added?

Biomedical data often contains missing values, visualising them explicitly within heatmaps (i.e. without imputation or defaulting to transparent which often visually clashes with the used colour scales) allows for proper communication of the data.

Mocks/Designs

See #1170

Notes

This feature has already been requested in in #975 and a PR with the necessary changes had previously been opened in #1170. While a workaround has been suggested, I believe this could hurt performance for large heatmaps and a new parameter would be more clean. Since the issue is still of concern, I created this new issue as requested here.

Inizia tracciando il punto di ingresso del rendering della heatmap in plotly.js e rivedendo l’implementazione precedente discussa in #1170. Verifica come vengono attualmente renderizzati i valori z NaN, quindi definisci una copertura che dimostri che un nancolor fornito viene applicato senza modificare i normali colori della heatmap o il comportamento.