I have done some further exploration of my problem. I think this may just be
a glitch with secondary axes line charts in Excel 2007.

I can show this using some "made-up" data. I entered the following data
into Excel 2007:

Date Series 1 Series 2
1 43 10
2 45 11
3 45 14
4 67 16
5 89 13
6 90 #N/A
7 89 #N/A
8 88 #N/A

I then used this data to create line chart. I made the axis a date axis.
And I put the second series on the second vertical axis.

I then changed the maximum date to "5" and Excel 2007 merely removes the
labels. It does not actually change the maximum date!

I note that if I just create the chart with Series 1 on it then I can change
the maximum date.

So it looks to me like it is a glitch! Or am I doing something wrong? Its
quite frustrating because I often use graphs with secondary axes.