plot.line fails when plot axis is a 1D coordinate

[TomNicholas]

MCVE Code Sample

import xarray as xr
import numpy as np

x_coord = xr.DataArray(data=[0.1, 0.2], dims=['x'])
t_coord = xr.DataArray(data=[10, 20], dims=['t'])

da = xr.DataArray(data=np.array([[0, 1], [5, 9]]), dims=['x', 't'],
                  coords={'x': x_coord, 'time': t_coord})
print(da)

da.transpose('time', 'x')

Output:

<xarray.DataArray (x: 2, t: 2)>
array([[0, 1],
       [5, 9]])
Coordinates:
  * x        (x) float64 0.1 0.2
    time     (t) int64 10 20

Traceback (most recent call last):
  File "mwe.py", line 22, in <module>
    da.transpose('time', 'x')
  File "/home/tegn500/Documents/Work/Code/xarray/xarray/core/dataarray.py", line 1877, in transpose
    "permuted array dimensions (%s)" % (dims, tuple(self.dims))
ValueError: arguments to transpose (('time', 'x')) must be permuted array dimensions (('x', 't'))

As 'time' is a coordinate with only one dimension, this is an unambiguous operation that I want to perform. However, because .transpose() currently only accepts dimensions, this fails with that error.

This causes bug in other parts of the code - for example I found this by trying to plot this type of dataarray:

da.plot(x='time', hue='x')

which gives the same error.

(You can get a similar error also with da.plot(y='time', hue='x').)

If the code which explicitly checks that the arguments to transpose are dims and not just coordinate dimensions is removed, then both of these examples work as expected.

I would like to generalise the transpose function to also accept dimension coordinates, is there any reason not to do this?

[dcherian]

I am unsure about this change but the plot statement does work on my end.

[TomNicholas]

Ah that's because I made a typo above. If you change it to da.plot(x='time', hue='x') then it should fail with master.

[TomNicholas]

So instead of changing transpose (which opens a can of worms) I've just made a small change in plot.line so that it can handle this case.