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.

先阅读 AnnotatedHeatmap 的实现，以及 issue 中链接的 annotated heatmap 文档/示例，然后比较 plotly.py issue 1300 中讨论的对比度行为。确定注释字体颜色可以在哪里根据单元格值进行调整，并在注释文本在各种 heatmap 颜色上都保持可读时完成。