[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.

Beginnen Sie damit, den Einstiegspunkt für das Heatmap-Rendering in plotly.js nachzuverfolgen und die zuvor in #1170 besprochene Implementierung zu prüfen. Bestätigen Sie, wie NaN-z-values derzeit gerendert werden, und definieren Sie anschließend eine Abdeckung, die zeigt, dass ein angegebenes nancolor angewendet wird, ohne normale Heatmap-Farben oder das Verhalten zu ändern.