Contour plot with norm and int or no levels fails.

[juseg]

MCVE Code Sample

import numpy as np
import xarray as xr
import matplotlib as mpl
data = xr.DataArray(np.arange(24).reshape(4, 6))
data.plot.contour(norm=mpl.colors.Normalize(vmin=5, vmax=10))

Yields:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../xarray/xarray/plot/plot.py", line 816, in plotmethod
    return newplotfunc(**allargs)
  File ".../xarray/xarray/plot/plot.py", line 694, in newplotfunc
    cmap_params, cbar_kwargs = _process_cmap_cbar_kwargs(
  File ".../xarray/xarray/plot/utils.py", line 772, in _process_cmap_cbar_kwargs
    cmap_params = _determine_cmap_params(**cmap_kwargs)
  File ".../xarray/xarray/plot/utils.py", line 278, in _determine_cmap_params
    cmap, newnorm = _build_discrete_cmap(cmap, levels, extend, filled)
  File ".../xarray/xarray/plot/utils.py", line 94, in _build_discrete_cmap
    n_colors = len(levels) + ext_n - 1

Expected Output

A plot.

Problem Description

I think this has to do with:

xarray/xarray/plot/utils.py

Line 744 in 95e4f6c

levels = 7 # this is the matplotlib default

And the line where the code fails:

xarray/xarray/plot/utils.py

Line 94 in 95e4f6c

n_colors = len(levels) + ext_n - 1

There should be an easy fix, I think.

Output of xr.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.8.1 (default, Jan 22 2020, 06:38:00)
[GCC 9.2.0]
python-bits: 64
OS: Linux
OS-release: 4.19.98-1-MANJARO
machine: x86_64
processor:
byteorder: little
LC_ALL: None
LANG: fr_CH.utf8
LOCALE: fr_CH.UTF-8
libhdf5: 1.10.5
libnetcdf: 4.7.3

xarray: 0.14.1
pandas: 0.25.3
numpy: 1.18.1
scipy: 1.4.1
netCDF4: 1.5.3
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: 1.0.4.2
nc_time_axis: None
PseudoNetCDF: None
rasterio: 1.1.1
cfgrib: None
iris: None
bottleneck: None
dask: 2.8.0
distributed: None
matplotlib: 3.1.2
cartopy: 0.17.0
seaborn: None
numbagg: None
setuptools: 44.0.0
pip: 19.3
conda: None
pytest: 5.3.4
IPython: 7.11.1
sphinx: 2.2.1

[mathause]

I think the problem is in _determine_cmap_params:

xarray/xarray/plot/utils.py

Line 262 in 95e4f6c

if levels is not None and norm is None:

This may have to be:

if levels is not None:

Introducing this change does not break any test (py.test xarray/tests/test_plot.py) and runs your example successfully. However, it then causes the same problem as in #3744.