Facetgrid: colors beyond range (extend) not saturated

[lvankampenhout]

Code Sample, a copy-pastable example if possible

Minimal example here:
https://github.com/lvankampenhout/bug-reports/blob/master/Facetgrid_cmap_extend.ipynb

Problem description

The extreme colors of neither the pcolormesh or colorbar (using extend='both') are not saturated as they should when faceting.

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.6.8 |Anaconda custom (x86_64)| (default, Dec 29 2018, 19:04:46) [GCC 4.2.1 Compatible Clang 4.0.1 (tags/RELEASE_401/final)] python-bits: 64 OS: Darwin OS-release: 17.7.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: C LANG: None LOCALE: None.None libhdf5: 1.10.1 libnetcdf: 4.4.1.1

xarray: 0.12.1
pandas: 0.23.4
numpy: 1.14.2
scipy: 0.18.1
netCDF4: 1.3.1
pydap: None
h5netcdf: None
h5py: 2.7.1
Nio: None
zarr: None
cftime: 1.0.0b1
nc_time_axis: None
PseudonetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: 1.2.0
dask: 0.13.0
distributed: None
matplotlib: 3.0.2
cartopy: 0.16.0
seaborn: 0.7.1
setuptools: 38.5.1
pip: 9.0.1
conda: 4.6.14
pytest: 3.0.5
IPython: 5.1.0
sphinx: 1.5.1

[mathause]

For completeness I add the minimal example

%matplotlib

import numpy as np; import xarray as xr
da = xr.tutorial.open_dataset('air_temperature').air
cmap = 'RdBu_r'
levels = np.arange(240,300+1,10)
da.isel(time=slice(4)).plot(cmap=cmap, col='time', levels=levels, extend='both')

What I think happens is that the cmap is determined twice. First in facetgrid.py. This returns a matplotlib.colors.ListedColormap with 6 colors. Then this reduced colormap is used in plot.py. However, now it only has 6 colors to choose from, so the _over and _under colors are the same as the first and last color.

[mathause]

The easiest way to fix this is probably to reset the cmap in facetgrid.py Lines 224-227

cmap = kwargs.get('cmap', None)

cmap_params, cbar_kwargs = _process_cmap_cbar_kwargs(func, kwargs, self.data.values)

# reset the colormap #GH2932
cmap_params['cmap'] = cmap

[lvankampenhout]

Thanks, I've implemented your suggestion as a workaround, but it fails with the following error:

NameError: name '_process_cmap_cbar_kwargs' is not defined

[dcherian]

See #3038

[dcherian]

Seems to work now? probably fixed by #3601