matplotlib issues in 19.01

[HoWol76]

We found two issues with matplotlib in conda3/analysis-19.01:

%matplotlib inline
import matplotlib.pyplot as plt
plt.plot([0,1],[0,1],color='C0') # works with analysis3-18.10, fails with analysis3-19.01

Error: ValueError: could not convert string to float: 'c0'

Also:

hw_dt is a xarray.core.dataarray.DataArray (f8)
hw_dt.plot() fails with:
AttributeError: module 'matplotlib.colors' has no attribute 'to_rgba'

These two might be related, might not be.