matplotlib deprecation warning
We use axes = fig.add_subplot()
once in the introduction.
As of version 3.1.1+, if the reader executes this line twice they will see a DeprecationWarning:
MatplotlibDeprecationWarning: Adding an axes using the same arguments as a previous
axes currently reuses the earlier instance. In a future version, a new instance
will always be created and returned. Meanwhile, this warning can be suppressed,
and the future behavior ensured, by passing a unique label to each axes instance.
Edited by Renato Alves