To add a secondary axis to a chart, first of all select the chart. After this, right-click anywhere on the chart area, and from the resulting menu click on ‘Insert/Delete Axes’. This will open the ‘Axes’ dialog box.
In this dialog box, we have two checkboxes under ‘Secondary Axes’. Check any one or both of these boxes as per the requirement and then click on ‘OK’. New secondary axis will get added to the chart as per the selection.