Colormap Normalisation Giving Unexpected/Incorrect Output

The behaviour when specifying “norm” for a colormap normalisation does not work as anticipated.

Below I use the example code from matplotlib and apply the same normalisation to the DataArray version of the data but get very different results.

MCVE Code Sample

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.colors as colors
import xarray 

#example from https://matplotlib.org/3.1.1/tutorials/colors/colormapnorms.html
#for colormap normalisation

N = 100
X, Y = np.mgrid[-3:3:complex(0, N), -2:2:complex(0, N)]
Z1 = np.exp(-X**2 - Y**2)
Z2 = np.exp(-(X - 1)**2 - (Y - 1)**2)
Z = (Z1 - Z2) * 2

fig, ax = plt.subplots(2, 1, figsize=(8, 8))
ax = ax.flatten()

bounds = np.linspace(-1, 1, 10)
norm = colors.BoundaryNorm(boundaries=bounds, ncolors=256)

ax[0].pcolormesh(X, Y, Z,
                       norm=norm,
                       cmap='RdBu_r')

#now add data into dataset and plot it using same normalisation
data = xarray.DataArray(Z, dims=('x', 'y'), coords={'x': X[:,0], 'y': Y[0,:]})
data.plot(ax=ax[1], x='x', y='y', norm=norm, add_colorbar=False)

plt.show()

Expected Output

Top is expected, bottom is actual

Problem Description

Colormap normalisation appears to be broken in xarray

Versions

Output of <tt>xr.show_versions()</tt>

xarray.show_versions()

INSTALLED VERSIONS

commit: None python: 3.7.3 | packaged by conda-forge | (default, Jul 1 2019, 21:52:21) [GCC 7.3.0] python-bits: 64 OS: Linux OS-release: 3.10.0-957.21.3.el7.x86_64 machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_GB.UTF-8 LOCALE: en_GB.UTF-8 libhdf5: 1.10.4 libnetcdf: 4.6.2

xarray: 0.15.1 pandas: 0.25.1 numpy: 1.17.2 scipy: 1.3.1 netCDF4: 1.5.1.2 pydap: None h5netcdf: None h5py: 2.9.0 Nio: None zarr: None cftime: 1.0.3.4 nc_time_axis: None PseudoNetCDF: None rasterio: 1.0.28 cfgrib: None iris: 2.2.0 bottleneck: None dask: 2.5.2 distributed: 2.5.2 matplotlib: 3.1.1 cartopy: 0.17.0 seaborn: 0.9.0 numbagg: None setuptools: 41.4.0 pip: 19.2.3 conda: None pytest: None IPython: 7.8.0 sphinx: None

[mathause]

If you use

norm = colors.BoundaryNorm(boundaries=bounds, ncolors=9)

it works. A new norm with ncolors=9 is built in _build_discrete_cmap - however, it is not used as norm is already defined:

https://github.com/pydata/xarray/blob/2542a63f6ebed1a464af7fc74b9f3bf302925803/xarray/plot/utils.py#L290-L292

The fix might be to use:

 if levels is not None or isinstance(norm, mpl.colors.BoundaryNorm): 
     cmap, norm = _build_discrete_cmap(cmap, levels, extend, filled) 

this breaks one test which is probably fixable (it tests that the norm is not changed when it is given).

Note that mpl seems to use a LinearSegmentedColormap while xarray creates a ListedColormap.

[huaracheguarache]

I also have an issue where xarray doesn’t produce the correct plot when normalizing with BoundaryNorm:

import xarray as xr
import matplotlib.pyplot as plt
import matplotlib.colors as colors
from cmcrameri import cm

airtemps = xr.tutorial.open_dataset("air_temperature")

# Convert to Celsius.
air = airtemps.air - 273.15
air.attrs = airtemps.air.attrs
air.attrs["units"] = "deg C"

# Select a timestep.
air2d = air.isel(time=500)

# Plotting discrete bounds with matplotlib works fine.
bounds = [x for x in range(-30, 31, 10)]
norm = colors.BoundaryNorm(boundaries=bounds, extend='both', ncolors=cm.vik.N)

fig, ax = plt.subplots()
cs = ax.pcolormesh(air2d.lon, air2d.lat, air2d, cmap=cm.vik, norm=norm)
fig.colorbar(cs)
plt.show()

# Plotting with xarray doesn't work.
fig, ax = plt.subplots()
air2d.plot.pcolormesh(ax=ax, norm=norm)
plt.show()

First one is from matplotlib:

Second one is from xarray:

I also get the following traceback after running the script:

Traceback (most recent call last):
  File "/home/michael/miniconda3/envs/testing_xarray/lib/python3.10/site-packages/matplotlib/cbook/__init__.py", line 287, in process
    func(*args, **kwargs)
  File "/home/michael/miniconda3/envs/testing_xarray/lib/python3.10/site-packages/matplotlib/backend_bases.py", line 3056, in mouse_move
    s = self._mouse_event_to_message(event)
  File "/home/michael/miniconda3/envs/testing_xarray/lib/python3.10/site-packages/matplotlib/backend_bases.py", line 3048, in _mouse_event_to_message
    data_str = a.format_cursor_data(data).rstrip()
  File "/home/michael/miniconda3/envs/testing_xarray/lib/python3.10/site-packages/matplotlib/artist.py", line 1282, in format_cursor_data
    neighbors = self.norm.inverse(
  File "/home/michael/miniconda3/envs/testing_xarray/lib/python3.10/site-packages/matplotlib/colors.py", line 1832, in inverse
    raise ValueError("BoundaryNorm is not invertible")
ValueError: BoundaryNorm is not invertible

Output of <tt>xr.show_versions()</tt>

INSTALLED VERSIONS

commit: None python: 3.10.0 | packaged by conda-forge | (default, Nov 20 2021, 02:25:18) [GCC 9.4.0] python-bits: 64 OS: Linux OS-release: 5.14.18-300.fc35.x86_64 machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_GB.UTF-8 LOCALE: (‘en_GB’, ‘UTF-8’) libhdf5: 1.12.1 libnetcdf: 4.8.1

xarray: 0.20.1 pandas: 1.3.4 numpy: 1.21.4 scipy: 1.7.2 netCDF4: 1.5.8 pydap: None h5netcdf: None h5py: None Nio: None zarr: None cftime: 1.5.1.1 nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: None distributed: None matplotlib: 3.5.0 cartopy: 0.20.1 seaborn: None numbagg: None fsspec: None cupy: None pint: None sparse: None setuptools: 59.2.0 pip: 21.3.1 conda: None pytest: None IPython: 7.29.0 sphinx: None