Plotting stuff example gives `ValueError'

[chenyulue]

I'm learning proplot according to the documents. When trying Plotting stuff example, it gives a ValueError: DiscreteNorm is not invertible.

My maplotlib and proplot version are as follows:

In [1]: matplotlib.__version__
Out[1]: '3.5.1'

In [2]: pplt.__version__
Out[2]: '0.9.5.post301'

I'm not sure whether this is related to the incompatibility with matplotlib of version 3.5.1.