issues: 735523592

``` 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:

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

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

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