Plotting norm is superseded by xarray vmin and vmax

[juseg]

Code Sample

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

# prepare array with values between 1e0 and 1e3
da = xr.DataArray(np.logspace(0, 3, 24).reshape(4, 6), dims=('x', 'y'))

# but we want to log-normalize between 1e0 and 1e2
norm = mcolors.LogNorm(1e1, 1e2)

# plot with log norm
da.plot.imshow(norm=norm)  # log norm is applied but bounds are replaced
#da.plot.imshow(norm=norm, vmin=norm.vmin, vmax=norm.vmax)  # works, but redundant

# show results
plt.show()

Problem description

When passing a norm argument to imshow etc, the norm is forwarded to matplotlib but its data bounds are replaced with those calculated by xarray.

Expected Output

I think that the norm's vmin and vmax should take precedence over xarray-generated vmin and vmax.

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 2.7.15.final.0 python-bits: 64 OS: Linux OS-release: 4.14.65-1-MANJARO machine: x86_64 processor: byteorder: little LC_ALL: None LANG: fr_CH.utf8 LOCALE: None.None

xarray: 0.10.8
pandas: 0.23.4
numpy: 1.15.0
scipy: 1.1.0
netCDF4: 1.4.1
h5netcdf: None
h5py: None
Nio: None
zarr: None
bottleneck: None
cyordereddict: None
dask: 0.18.2
distributed: None
matplotlib: 2.2.3
cartopy: 0.15.1
seaborn: None
setuptools: 40.0.0
pip: 18.0
conda: None
pytest: None
IPython: 5.8.0
sphinx: 1.7.6

[shoyer]

Thanks for bringing this up!

@dcherian @fmaussion I'm going to defer to your judgment here...

[dcherian]

Yes this looks sensible to me and is an easy fix I think.

@juseg PR welcome!

[juseg]

I was working on it but you were quicker, thanks!

[dcherian]

Ooops. Sorry! @juseg

[dcherian]

@juseg can you take a look at #2443?

[juseg]

@dcherian Thanks! This works for me.

I appreciate that my current code using the workaround plot(norm=norm, vmin=norm.vmin, ...) still works. To be fully consistent I suggest to replace the error message Cannot supply vmin and a norm with vmin with Cannot supply vmin and a norm with a different vmin (idem with vmax).

Edit: I just checked that matplotlib's imshow accepts imshow(norm=norm, vmin=vmin, vmax=vmax), with the user-provided vmin taking precedence over the norm. So I'm not sure that xarray need to raise an error here.