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.

先閱讀 AnnotatedHeatmap 的實作，以及 issue 中連結的 annotated heatmap 文件／範例，然後比較 plotly.py issue 1300 中討論的對比度行為。確定註解字型顏色可以在哪裡根據儲存格值進行調整，並在註解文字於各種 heatmap 顏色上都保持可讀時完成。