~matplotlib.axes.Axes.twinx and ~matplotlib.axes.Axes.twiny now accept a
delta_zorder keyword argument, a relative offset added to the original Axes'
zorder, to control whether the twin Axes is drawn in front of, or behind, the
original Axes. For example, pass delta_zorder=-1 to easily draw a twin Axes
behind the main Axes.
In addition, Matplotlib now automatically manages background patch visibility
for each group of twinned Axes so that only the bottom-most Axes in the group
has a visible background patch (respecting frameon).