It’s pretty easy to add a secondary axis to a graph if you’d like to plot two different series of data that had different scales or units. Generally you only want to have graphs of data with different units, if you’re trying to show the relationship between the two series, so use this tip sparingly.
So I’ve set up a quick demo, using fake data of a personal library. This is what usually happens if you try to graph percentages and values on the same graph. It’s ugly, so we’ll need add a secondary axis for the % Science Fiction series. Here’s the finished Excel file, so you can follow along: Secondary Axis Excel File
So Right Click the line that you want to be plotted on the secondary series. Now click on the “Format Data Series” option, and a new box should pop up. Under the “Series Options” tab, you will be able to set the data to be plotted on the secondary axis.
And here’s the result.