colormap scaling heatmap

[PhilippFW]

cool package, love the visualization

I am using the heatmap to visualize variant frequencies (and not enrichment scores). While I can recenter the colormap by using colorbar_scale=(lower, upper) I noticed that the colormap is centered around the mean (i.e. (lower+upper)/2). Thius distorts the colormap. Since I am looking at frequencies, I want a linear colorscale between lower and upper. How to do that?

i.e. it should be:
upper = 1e-3
middle=1e-4
lower=1e-5