There are two main approaches to adding a secondary axis to a chart in Excel:

Method 1: Using the Chart Design Menu (For Excel 2010 and Later)

1. Select your chart: Click anywhere on the chart to activate the Chart Tools tabs on the ribbon.
2. Change chart type (optional): If your data isn't already in a suitable chart type (e.g., combo chart), you can use this opportunity to switch it. Go to the "Chart Design" tab and click the "Change Chart Type" button. Select "Combo" from the left pane and choose a chart type that allows a secondary axis, such as "Cluster Column - Line on Secondary Axis."
3. Format the data series: Click on the data series you want to display on the secondary axis. This will typically be the line representing values that differ in scale from the other data series.
4. Activate the secondary axis: Right-click on the selected data series and choose "Format Data Series." In the right pane that opens, go to the "Series Options" tab. Under "Plot Series On," select "Secondary Axis."
5. (Optional) Customize the secondary axis: You can format the secondary axis by right-clicking on it and selecting "Format Axis." This allows you to change its scaling, labels, and appearance to better match your secondary data.

Method 2: Using the Select Data Menu (For all Excel versions)

1. Select your chart: Click anywhere on the chart to activate the Chart Tools tabs.
2. Edit data selection: Go to the "Chart Design" tab and click the "Select Data" button.
3. Edit data series: In the "Select Data Source" window, click the "Add" button to add a new data series specifically for the secondary axis. Define the data range for this series.
4. Set secondary axis: In the "Legend Entries (Series)" section, select the checkbox next to the newly added data series. Then, click the "Format Selection" button. In the "Format Data Series" pane, go to the "Series Options" tab and under "Plot Series On," choose "Secondary Axis."
5. (Optional) Customize the secondary axis: Similar to method 1, you can right-click on the secondary axis and format it using the "Format Axis" option.

By following these steps, you can effectively add a secondary axis to your chart, allowing you to visualize two sets of data with different scales on the same chart.