[BUG/ISSUE] Colorbar ticks are incorrectly placed if the plot is zero or undefined everywhere.

[yantosca]

Describe the bug

It seems that newer versions of Matplotlib (since 3.0 or 3.1) cause colorbar ticks to be placed incorrectly for plots that are zero everywhere or undefined everywhere

This is probably caused in the normalization of colors in the from the min and max of the data range to the matplotlib color scale.

[yantosca]

This issue is now resolved in 1c7647a. We now test whether or not we are using matplotlib 3.1.1 or an older version, and adjust the tickmark placement accordingly.

[yantosca]

I have now pushed commit: f450fe2, which definitively fixes this issue.

The problem was not so much matplotlib 3.1.1 vs. older versions,. Instead, if we use the RdBu colortable (as we do for diff-of-diffs plots, like the one shown above), then we must take care to place the tick in the middle of the colorbar if Ref and/or Dev is zero or undefined everywhere. I modified the code in the if all_zero or all_undefined blocks accordingly.

        if all_zero or all_undefined:
            if use_cmap_RdBu:
                cb.set_ticks([0.0])
            else:
                cb.set_ticks([0.5])

This now produces the proper tick placement for RdBu difference color map:

As well as the non-difference color map WhGrYlRd: