staggered gridlines
 

I have a line chart with multi-level category labels on the x-axis. I want
to have gridlines for every BIG category division but not for every small
category division. In other words, there should be a gridline for every 5th
(small) category or so. This was easy in Office 2003, but when I opened
these charts in 2007, there was a gridline for EVERY category and seemingly
no way to change it. Changing options like "Intervals between labels" and
"Intervals between tick marks" doesn't do anything when you have multi-level
categories.