plotting facet grid with singleton dimension should create a facet grid with size 1

[liorella-qm]

What is your issue?

In my application, I want to plot facet grids of datasets, and the dimension I want to facet sometimes has a length of 1 and sometimes more than 1. When the length of the dimension is > 1, the plot(col=....) method of DataArray works as expected. However, when it is 1, I get an error and my code fails, although it would have been natrual for the plot method to create a FacetGrid with length of one facet.

As a minimal example, this code:

import xarray as xr
da = xr.DataArray(np.random.rand(3,1), dims=('x', 'y'))
da.plot(col='y')

fails with an error:

KeyError: 'y'

This example works as expected:

import xarray as xr
da = xr.DataArray(np.random.rand(3,2), dims=('x', 'y'))
da.plot(col='y')

[headtr1ck]

This is caused by the squeeze() here:

xarray/xarray/plot/dataarray_plot.py

Line 267 in 4194920

darray = darray.squeeze().compute()

@Illviljan do you know why this is required? I assume you want to remove singleton dimensions to better choose the correct plot method?

Maybe we should exclude col, row dims from the squeeze?
something like:
darray.squeeze(d for d, s in darray.sizes.items() if s == 1 and d not in (row, col, hue))

[headtr1ck]

@liorella-qm in the meantime you can use da.plot.line(col='y') as a workaround

[dcherian]

See #620 (!)

Maybe we should exclude col, row dims from the squeeze?

This seems sensible.

[krokosik]

Since I didn't see anyone objecting to the solution provided by @headtr1ck and I also found this behaviour annoying, I hope that nobody minds that I created a PR with it.