Twin Axes delta_zorder

twinx and 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).