pivot chart gridlines
 

I need to make major gridlines between the outer of my two X axis groupings
in a pivotchart. In other words, I have 2 levels of rows in my table, which
translate into an "upper" and "lower" axis label for the chart. I want to
have gridlines that show the divisions between the lower axis label (the
ones that occur less frequently).

Any way to do this? I am using '07. When I've tried to select the axis, it
is treated as one general one, not as two sets of grouping. So my problem is
how to isolate the one of the groupings and indicate I want to have gridlines
for that.
--
Boris