How to Add a Secondary Axis in Excel
Excel’s charts give you a variety of ways to visualize your data. For charts that utilize an X-and-Y-axis layout, you have the ability to view a series of data, which allows you to compare two different things, but those things usually have the same unit of measure. A secondary axis is used to compare two things that don’t have the same unit of measure.
When You Should Use a Secondary Axis in Excel
Consider this example showing a company’s revenue versus its expenses for the last five years. Both of these are measured in dollars, so we can set up a line graph showing both of these in the same view to determine the relationship. As you can see, the y-axis on the left is displaying the USD units, but what if you want to see if there’s a similar trend as it pertains to costs and employees? The unit for Employees is people, not dollars, so you can’t very well use the existing y-axis. This is where you should add a second axis to ensure your reader can accurately understand what the numbers mean. A second reason is when the two series don’t have data in the same magnitude. Consider, for example, the revenue of the company versus the employees. The chart above shows that when visualized together it doesn’t provide much insight, because the number of employees is so low you can’t determine what’s happening with it. Instead, you can add a second axis that has both its own units as well as its own scale, allowing you to really compare the two.
