plot.line breaks depending on coordinate shape

[apatlpo]

plot.line breaks depending on coordinate shape, see the code below:

x = np.arange(10)
y = np.arange(20)

ds = xr.Dataset(coords={'x': x, 'y': y})
#ds = ds.assign_coords(z=ds.y+ds.x) # goes through
ds = ds.assign_coords(z=ds.x+ds.y) # breaks
ds['v'] = (ds.x+ds.y)
ds['v'].plot.line(y='z', hue='x')

This breaks with the following error:

...
~/.miniconda3/envs/equinox/lib/python3.7/site-packages/matplotlib/axes/_base.py in _plot_args(self, tup, kwargs)
    340 
    341         if x.shape[0] != y.shape[0]:
--> 342             raise ValueError(f"x and y must have same first dimension, but "
    343                              f"have shapes {x.shape} and {y.shape}")
    344         if x.ndim > 2 or y.ndim > 2:

ValueError: x and y must have same first dimension, but have shapes (20, 10) and (10, 20)

I would have expected that that dimension order would not matter

Versions

Output of `xr.show_versions()`

INSTALLED VERSIONS

commit: None
python: 3.7.6 | packaged by conda-forge | (default, Mar 23 2020, 23:03:20)
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 3.12.53-60.30-default
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
libhdf5: 1.10.5
libnetcdf: 4.7.4

xarray: 0.15.1
pandas: 1.0.3
numpy: 1.18.1
scipy: 1.4.1
netCDF4: 1.5.3
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: 2.4.0
cftime: 1.1.1.2
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2.13.0
distributed: 2.13.0
matplotlib: 3.2.1
cartopy: 0.17.0
seaborn: 0.10.0
numbagg: None
setuptools: 46.1.3.post20200325
pip: 20.0.2
conda: None
pytest: None
IPython: 7.13.0
sphinx: None

[TomNicholas]

Thanks for the clear bug report @apatlpo ! I've opened a fix in #3934.

[apatlpo]

thanks to you for the fix @TomNicholas !