I've been able to specify the placement of overlapping plot objects by controlling the order of calls in the DrawGraph() routine. By that I mean that the second plot call is drawn on top of the first plot, the third goes on top of the second and so forth.
But the grid always appears on top of all the plot objects. I'd prefer to have the grid to be "below" all plot objects.
The grids work a little differently than the other chart objects. The default we use always places the grid on top of the objects. You can change this behavior using the WGToggleGridsOrder function. See the Charting Tools manual.