I was a bit confused by Del's and David's posts, and thought you might be as
well.
You need at least two series in a chart to enable secondary axes. To get
secondary axes, double click one of the series, and on the Axis tab of the
Format Series dialog, choose Secondary. Most of the time, this provides only
a secondary Y axis. To control which axes appear in the chart, go to Chart
menu > Chart Options, click on the Axes tab, and check the various primary
and secondary axes you need. For example, you would uncheck the secondary Y
axis box, but check the secondary X axis box. If only one X axis or one Y
axis is selected, all series use the selected axis, and the other doesn't
appear.
- Jon