Hi Peter,
Create you xy scatter chart. Use the source data dialog to add another
data series. The data series requires 12 values.
Select the new series and move to the secondary axis. Now change its
chart type to column. Use the source data dialog again to specify the
category label range. Remove the tick labels from the main x axis.
Use Chart options to display the secondary x axis. You should now have
the month names at the top of the chart.
Double click secondary xaxis and check on the scale tab, Value (Y) axis
crosses between categories. Double click secondary yaxis and on the
scale tab uncheck Category(X) axis crosses at maximum value.
You can now delete the secondary yaxis. Format the columns to have no
border and fill.
Or you could use a dummy data series to create you own xaxis labels.
See Jon's examples of arbitrary axis scales to get an idea.
http://peltiertech.com/Excel/Charts/axes.html#ArbAxis
Cheers
Andy