zorder not working for grid lines??...

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.)

Here is code snippet for grid stuff....

        grid_ = plot_.get_xgridlines() + plot_.get_ygridlines()
        pylab.setp(grid_, linestyle = "-", color = "k",
                   linewidth = "0.5",
                   zorder = 0.5)