How can I add an alternate scale to my chart?

Click on the graph which you would like to display on an alternate scale. On the Presentation properties tab, in the Content group, tick the checkbox for Alternate scale.

You can also access this setting in the Graph Layout window.

Please note that for alternate scale to work the axis synchronization cannot be set to the option Values. To check this, click on either of the axes, on the Presentation properties tab in the Axis group you will find the Synchronize options.

For more information:

Adding an alternate scale