pass a figure, and have the colorbar fit all plot objects on all axes of the figure
That would be nice if all plots are to share the same colorbar indeed.
But maybe it could be possible to do the reverse:
define the cb=Colorbar(...) first
and pass a colorbar=cb argument to all heatmaps that should share this colorbar ?
That would suppress the need to define a reference for each heatmap,
while allowing to have separate groups of heatmaps with their own colorbar if needed.