If I'm not mistaken, zorder determines what objects will be
> "on top" of what other objects.
> I got this working fine for some lines on my plot but it
> seems to have no effect on **grid lines**. (I want grid
> lines to be *behind* everything.)
gridlines are part of the Axis, and do not obey the zorder. But you
can put them above or below by setting the axisbelow property of the
Axes
ax.set_axisbelow(True)
JDH