TYP: annotate plotting based on _get_axe_freq
In some places in plotting `ax` is an Axes object and in other its an Axis object.  Current goal is to pin these down.

in timeseries._get_ax_freq we call `ax.get_shared_x_axes()`, which is an Axes method that does not exist on Axis.  This annotates that usage and annotates all the other places where we can infer Axes from that.
Thanks @jbrockmendel 