TYP: Annotate plotting stacker
In AreaPlot._plot we call `ax.fill_between`, which means `ax` must be an `Axes` object (and in particular, not an `Axis` object).  This chases down all the other places we can infer `Axes` from that.

Then some edits in an `__init__` to get mypy passing, and revert annotations of `_plot` in a few places because mypy complained about signature mismatch.