There are a few ways to add a secondary axis in Excel. One way is to add the secondary axis as a data series. To do this, select the data that you want to plot on the secondary axis and then click on the "Chart Tools" tab on the ribbon. Then, click on the "Design" tab and select the "Series" option. From there, select the "Secondary Axis" checkbox.

Another way to add a secondary axis is to use the "Axes" option. To do this, select the data that you want to plot on the secondary axis and then click on the "Chart Tools" tab on the ribbon. Then, click on the "Layout" tab and select the "Axes" option. From there, select the "Secondary Axis" checkbox.