CLN: plotting code

[jbrockmendel]

Nothing serious: improve some typing, de-duplicate, hopefully-helpful comments, remove a couple unreachable paths.

[mroeschke]

Thanks @jbrockmendel

[jorisvandenbossche]

Some typing errors because of this:

  /home/runner/work/pandas/pandas/pandas/plotting/_matplotlib/converter.py:236:31 - error: Cannot access attribute "freq" for class "Axis"
    Attribute "freq" is unknown (reportAttributeAccessIssue)
  /home/runner/work/pandas/pandas/pandas/plotting/_matplotlib/converter.py:280:35 - error: Cannot access attribute "ordinal" for class "NaTType"

They were hidden because of the doctest failure that is present on main at the moment (I am fixing that)