They've added this is 2007, which has so far only added to the confusion. It
makes no sense to have two sets of Y axis gridlines, so set up the chart
with the gridline-requiring data on the primary axis.
Although not used ofter, I do have a need to have two separate gridlines. I have some values going negative and others going positive. I chart the negative values in one column (start at 0 and go down) and then have the positive numbers start from the negative base (say -37) and create another column going up. The secondary gridlines allow the user to easily get the value of the positive numbers.