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
735523592,MDU6SXNzdWU3MzU1MjM1OTI=,4562,Cannot plot multiindexed (stacked) coordinate as hue variable ,20617032,open,0,,,7,2020-11-03T17:53:01Z,2021-11-16T23:39:09Z,,NONE,,,,"```
import numpy as np
import pandas as pd
import xarray as xr

data = np.random.rand(50,5)

x_idx = np.linspace(0, 50)

mi_idx1 = ['a','b','c','d','e']
mi_idx2 = [1,2,3,4,5]

mi = pd.MultiIndex.from_arrays([mi_idx1,mi_idx2], names=['mi_idx1', 'mi_idx2'])

coords = {
    'x': x_idx,
    'mi': mi
}

da = xr.DataArray(data, coords=coords, dims = ['x', 'mi'])

da.plot(hue='mi')
```


It appears since version 0.16.0, that plotting with a multindex coordinate as a hue dimension no longer works. In version 0.15.1 I get a plot like this:

![xarray_issue](https://user-images.githubusercontent.com/20617032/98021886-ba7f6b00-1db9-11eb-80af-bea7540cc785.png)

However, when upgrading to 0.16.0 or 0.16.1 I get the following traceback:

```

ValueError                                Traceback (most recent call last)
<ipython-input-1-2de8705a6ec5> in <module>
     19 da = xr.DataArray(data, coords=coords, dims = ['x', 'mi'])
     20 
---> 21 da.plot(hue='mi')

~\anaconda3\envs\datanalysis\lib\site-packages\xarray\plot\plot.py in __call__(self, **kwargs)
    444 
    445     def __call__(self, **kwargs):
--> 446         return plot(self._da, **kwargs)
    447 
    448     # we can't use functools.wraps here since that also modifies the name / qualname

~\anaconda3\envs\datanalysis\lib\site-packages\xarray\plot\plot.py in plot(darray, row, col, col_wrap, ax, hue, rtol, subplot_kws, **kwargs)
    198     kwargs[""ax""] = ax
    199 
--> 200     return plotfunc(darray, **kwargs)
    201 
    202 

~\anaconda3\envs\datanalysis\lib\site-packages\xarray\plot\plot.py in line(darray, row, col, figsize, aspect, size, ax, hue, x, y, xincrease, yincrease, xscale, yscale, xticks, yticks, xlim, ylim, add_legend, _labels, *args, **kwargs)
    293 
    294     ax = get_axis(figsize, size, aspect, ax)
--> 295     xplt, yplt, hueplt, xlabel, ylabel, hue_label = _infer_line_data(darray, x, y, hue)
    296 
    297     # Remove pd.Intervals if contained in xplt.values and/or yplt.values.

~\anaconda3\envs\datanalysis\lib\site-packages\xarray\plot\plot.py in _infer_line_data(darray, x, y, hue)
     66 
     67         if y is None:
---> 68             xname, huename = _infer_xy_labels(darray=darray, x=x, y=hue)
     69             xplt = darray[xname]
     70             if xplt.ndim > 1:

~\anaconda3\envs\datanalysis\lib\site-packages\xarray\plot\utils.py in _infer_xy_labels(darray, x, y, imshow, rgb)
    378         y, x = darray.dims
    379     elif x is None:
--> 380         _assert_valid_xy(darray, y, ""y"")
    381         x = darray.dims[0] if y == darray.dims[1] else darray.dims[1]
    382     elif y is None:

~\anaconda3\envs\datanalysis\lib\site-packages\xarray\plot\utils.py in _assert_valid_xy(darray, xy, name)
    410     if xy not in valid_xy:
    411         valid_xy_str = ""', '"".join(sorted(valid_xy))
--> 412         raise ValueError(f""{name} must be one of None, '{valid_xy_str}'"")
    413 
    414 

ValueError: y must be one of None, 'mi_idx1', 'mi_idx2', 'x'

```

Setting x='x' does not fix the problem

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

INSTALLED VERSIONS
------------------
commit: None
python: 3.8.3 (default, Jul  2 2020, 17:30:36) [MSC v.1916 64 bit (AMD64)]
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 142 Stepping 12, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: English_United States.1252
libhdf5: 1.10.4
libnetcdf: 4.7.3

xarray: 0.16.0
pandas: 1.0.5
numpy: 1.18.5
scipy: 1.5.0
netCDF4: 1.5.3
pydap: None
h5netcdf: None
h5py: 2.10.0
Nio: None
zarr: None
cftime: 1.2.1
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: 1.3.2
dask: 2.20.0
distributed: 2.20.0
matplotlib: 3.2.2
cartopy: None
seaborn: 0.10.1
numbagg: None
pint: 0.16.1
setuptools: 49.2.0.post20200714
pip: 20.1.1
conda: None
pytest: 5.4.3
IPython: 7.16.1
sphinx: 3.1.2
</details>
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