SymLogNorm colorbar incorrect on master

[greglucas]

Bug report

Bug summary

The colormap/tick locations within the colorbar are not being updated properly with a SymLogNorm on master. I think this is different than #16269 because it is actually creating the colorbar, just a wrong one.

Compare the previous correct version:
https://matplotlib.org/3.1.0/gallery/userdemo/colormap_normalizations_symlognorm.html

with a fresh checkout of master:

Difference in the colorbar of the top axis.

[jklymak]

That example is pretty broken anyways.

Colorbars changed to behave more like normal axes, though I thought they changed in 3.1. Can you bisect this and see what commit made this change?

I'm ill-disposed to SymLogNorm, but maybe its an easy fix.

[dstansby]

@jklymak good job I'm well-disposed 😉 - I'll try to look into this as I think I have the same issue too!

[dstansby]

Bisects to dc51a68

[jklymak]

Hmm, well not surprising. I guess the hand-made axes handling that used to be in colorbar did something special with SymLogNorm. This comes back to the issue that @anntzer has that scales and norms are not terribly different. I suspect the correct fix is for someone to gin up a SymLogScale if it doesn't already exist and get that to work and then let it be applied to colorbars. Or special case SymLogNorm in colorbar, but thats a last resort IMHO.

[greglucas]

@dstansby, out of curiosity how did you bisect that so fast? When I do a git bisect and rebuild with python -mpip install . it takes quite a while to rebuild mpl each time. Is there an automated way that you've got to make bisecting faster?

[jklymak]

You don’t need to pip install each time if you don’t need to rebuild any of the c code. (And you’ve used pip install -e)

[anntzer]

Some more tricks (everything here assumes you're in a -e install):

• Even if you need to rebuild c extensions you can just do python setup.py build_ext --inplace which will just rebuild the extensions and skip the rest.
• ccache (https://ccache.dev/) really helps too (because if you're bisecting even only once in a while, you'll be compiling the same files again and again).

[jklymak]

So it seems that SymLogNorm itself is also broken, so perhaps not such a bad thing that its colorbar doesn't work: #16376 But of course we should fix these things ASAP.

[tacaswell]

@greglucas Using pip install -e . is one way to make it faster (as you don't have to actually re-install if only the python changes) and the other is to use ccache so that the c-extensions build almost instantly.

[greglucas]

Thanks for the helpful hints on commands! A quick search through the docs doesn't indicate anything telling me about ccache and the wording is pretty vague around the pip install -e . command. I think this would actually be helpful for other new contributors getting started and bisecting, so I will try and look into adding it somewhere in the docs.