Allow setting zmid with zmin and zmax in plotly.graph_objects.Heatmap

Feature request

I've been trying to plot a subset of a ranked list (gene differential expression dataset). I would like the minimum and maximum color scale set to that of the original complete ranked list, with the midpoint set to zero. It currently seems to be impossible in plotly to do this. I can plot the subset with the colorscale min and max set to the original dataset min and max, but this causes the midpoint to no longer be set to zero.

It can be desirable to set a colorscale to the minimum and maximum of a positive/negative spanned dateset with unequal absolutes but to still have the midpoint at zero without needing to construct and entirely custom colorscale. zmid being ignored when zmax and zmin are set just seems a super odd behavior to me.

从 plotly.graph_objects.Heatmap 入手，跟踪 zmin、zmax 和 zmid 如何被处理以用于 colorscale。检查现有的 Heatmap 测试，确认相关行为。完成标准是：即使明确设置了 zmin 和 zmax，zmid 仍然有效，并覆盖正负边界不相等的情况。