Major gridlines - worked great on Excel 2003 but not 2007

Hi, I have a line chart that plots quarterly progress over the life of a
project. In Excel 2003 it looked so nice, with every fourth quarter vertical
line being a bold vertical gridline, while the quarters inbetween were
visible, but not bold. It was therefore very easy to see where each new year
began due to the bold vertical gridline.

However in 2007, all vertical gridlines have same weight. I cannot get this
graph to do in 2007 what it did so nicely in 2003. I've tried everything I
can under Axis Options and in the Chart Layout menu.

Any ideas would sure be appreciated.
Harold