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
769348008,MDU6SXNzdWU3NjkzNDgwMDg=,4702,Plotting in 2D with one non-dimension coordinate given behaves contrary to documentation and can cause cryptic ValueError,38358698,open,0,,,2,2020-12-16T23:33:25Z,2020-12-17T18:35:13Z,,CONTRIBUTOR,,,,"**What happened**:

I called a 2-D plot method with one coordinate specified as a non-dimension coordinate through the `x` or `y` argument. The method raised a `ValueError` that complained about a tuple of duplicate coordinates that I had not provided.

Also, when I specified `x` and omitted `y`, the method took `y` to be the second dimension of the `DataArray`.


**What you expected to happen**:

I expected the unspecified `x` or `y` argument to behave as when I specified neither argument, so that, for a suitable `DataArray` `da`, calling `da.plot(x=""lon"")` would have the same *Y* axis as `da.plot()`. That expectation is consistent with the docstring common to 2-D plotting methods, where the default values for the `x` and `y` arguments are `da.dims[1]` and `da.dims[0]`, respectively. However, as discussed below, the docstring does not mention that *xarray* tries to guess the omitted coordinate.


**Minimal Complete Verifiable Example**:

Executing

```python
import numpy as np
import xarray as xr

da = xr.DataArray(
    np.arange(20).reshape(4, 5),
    dims=(""v"", ""u""),
    coords={""lat"": (""v"", np.linspace(0, 30, 4)), ""lon"": (""u"", np.linspace(-20, 20, 5))},
)
da.plot(x=""lon"")
```

causes (with file paths abbreviated)

```
Traceback (most recent call last):
  File ""<stdin>"", line 1, in <module>
  File ""...\xarray\plot\plot.py"", line 446, in __call__
    return plot(self._da, **kwargs)
  File ""...\xarray\plot\plot.py"", line 200, in plot
    return plotfunc(darray, **kwargs)
  File ""...\xarray\plot\plot.py"", line 707, in newplotfunc
    darray = darray.transpose(*dims, transpose_coords=True)
  File ""...\xarray\core\dataarray.py"", line 2036, in transpose
    dims = tuple(utils.infix_dims(dims, self.dims))
  File ""...\xarray\core\utils.py"", line 725, in infix_dims
    raise ValueError(
ValueError: ('u', 'u') must be a permuted list of ('v', 'u'), unless `...` is included
```

Executing `da.plot(y=""lon"")` causes the same exception.


**Anything else we need to know?**:

Two aspects of this issue that I see are the behavior and the documentation.

The behavior in the above example involves *xarray* trying to guess the other dimension [#1291 in v. 0.9.2]. The [guessing logic](https://github.com/pydata/xarray/blob/778a16ef392edd13cd14e1548700842e53e4656f/xarray/plot/utils.py#L379-L384) tests whether the given coordinate equals one of the array's dimension names; here, the given coordinate is a non-dimension coordinate and fails to match. When the match fails, the other coordinate is set to the second dimension, whether the given coordinate was `x` or `y`. One possible improvement would be to set the `y` coordinate to the first dimension instead, as in the following replacement for the last line of the guessing logic:

```python
y = darray.dims[1] if x == darray.dims[0] else darray.dims[0]
```

With that change, `da.plot(x=""lon"")` and `da.plot(y=""lat"")` would succeed and produce the plots I expect, while `da.plot(x=""lat"")` and `da.plot(y=""lon"")` would raise `ValueError`. Also, I would find it more helpful if the exception message were more clearly related to the arguments of the call. The `_infer_xy_labels` function might need to notice that it was about to set `x` and `y` to the same dimension name and raise an exception at that point.

The [documentation](https://github.com/pydata/xarray/blob/778a16ef392edd13cd14e1548700842e53e4656f/xarray/plot/plot.py#L486-L489) seems not to have been updated for #1291 and does not mention the guessing behavior.

**Environment**:

<details><summary>Output of <tt>xr.show_versions()</tt></summary>

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.8.5 (default, Sep  3 2020, 21:29:08) [MSC v.1916 64 bit (AMD64)]
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: English_United States.1252
libhdf5: 1.10.4
libnetcdf: 4.7.3

xarray: 0.16.1
pandas: 1.1.3
numpy: 1.19.2
scipy: None
netCDF4: 1.5.3
pydap: None
h5netcdf: 0.8.1
h5py: 2.10.0
Nio: None
zarr: None
cftime: 1.3.0
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2.30.0
distributed: 2.30.1
matplotlib: 3.3.2
cartopy: None
seaborn: None
numbagg: None
pint: None
setuptools: 50.3.0.post20201006
pip: 20.2.4
conda: None
pytest: None
IPython: 7.18.1
sphinx: None
```
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