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.

まず AnnotatedHeatmap の実装と、issue にリンクされている annotated heatmap のドキュメント／例を読み、その後 plotly.py issue 1300 で議論されているコントラストの挙動と比較します。注釈のフォント色がセルの値を考慮できる箇所を特定し、heatmap の色全体にわたって注釈テキストが読みやすい状態になったら完了します。