id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
497184021,MDU6SXNzdWU0OTcxODQwMjE=,3334,plot.line fails when plot axis is a 1D coordinate,35968931,closed,0,,,3,2019-09-23T15:52:48Z,2019-09-26T08:51:59Z,2019-09-26T08:51:59Z,MEMBER,,,,"#### MCVE Code Sample
<!-- In order for the maintainers to efficiently understand and prioritize issues, we ask you post a ""Minimal, Complete and Verifiable Example"" (MCVE): http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports -->

```python
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:
```python
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](https://github.com/pydata/xarray/pull/2556/files?file-filters%5B%5D=.py#diff-ffd3597671590bab245b3193affa62b8R1437) 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?

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