Michael -
In order to use the secondary axis, you need to have one or more series
assigned to the secondary axes. You could use one of your data series
for this (or any dummy axis).
Alternatively, you could use a dummy series to simulate another axis as
described here:
http://peltiertech.com/Excel/Charts/ArbitraryAxis.html
http://peltiertech.com/Excel/Charts/TertiaryAxis.html
Of course, you need to align the fake axis along the top, not left, of
the plot area. It's a little more work, but this technique is
particularly helpful if the conversion is not regular, or if offsets
prevent display of "natural" units. For example, if you plot 30 to 80°F
on the primary X axis, the secondary axis scales from -1 to 27°C
(rounding to the nearest degree). There's no way to get it to display
ticks from 0 to 25°C every five degrees using a "real" axis, but the
dummy axis technique lets you put axis ticks and labels wherever you want.
- Jon