matplotlib.colors throws ValueError when a list of colors is passed to xr.DataArray.plot.line

MCVE Code Sample

```python import matplotlib import matplotlib.colors import matplotlib.cm import xarray as xr import numpy as np

data = np.random.random((10,10)) x = np.arange(0,1.0,0.1) y = np.arange(0,100,10.0)

norm = matplotlib.colors.Normalize(vmin=y.min(),vmax=y.max()) cmapper = matplotlib.cm.ScalarMappable(norm=norm,cmap=matplotlib.cm.viridis).to_rgba

da = xr.DataArray(data,dims=['x','y'],coords={'x':x,'y':y})

da.plot.line(x='x',color=cmapper(da.y.values)) #throws ValueError in matplotlib/colors.py

da.to_pandas().plot(color=cmapper(da.y.values)) ```

Expected Output

Problem Description

I need to be able to map coordinate values to colors so that data plotted from different arrays are always aligned in colorspace. Depending on the data, specific y-values may be missing so a simple color cycle doesn't work here unfortunately. Passing a list of colors to pandas.plot seems to work but not for xarray.plot. While the obvious workaround is to cast to pandas, is there something I can do to make this work for xarray?

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