A dual axis chart or multiple axes chart uses two axes to easily illustrate the relationships between two variables with different magnitudes and scales of measurement.
The relationship between two variables is referred to as correlation.
View more examples of Dual Axis Charts.
Infogram is a free online chart maker that offers four different double axis charts (Column and Line Chart, Dual Line Chart, Grouped Column and Line Chart and Stacked Column and Line Chart):
Dual Line Chart – This dual axis chart compares two line charts. There can be more than two lines if need be.
Grouped Column and Line Chart - This dual axis chart combines a grouped column chart with a line chart.
Stacked Column and Line Chart - This dual axis chart combines a stacked column chart with a line chart.
With a dual axis chart, you are essentially combining multiple charts and adding a second y-axis for comparison. Some members of the data visualization community are skeptical about the use of dual axis charts because they can often be confusing, poorly designed, and potentially misleading for the viewer.
However, if you have limited space and want to quickly establish the relationship between two variables, the dual axis chart might be the right fit for you.
Make sure your y-axes are related – Dual axis charts can be useful when comparing values that have different units of measurement because the things they measure are somehow related in a meaningful way. So make sure both axes are helping you tell your story effectively.
Place primary y-axis on the left – We are hard-wired to look at the y-axis on the left first. Make sure your more important variable is placed there.
Use contrasting colors – Color code your dual axis chart to make it easier for people to understand the data sets you have plotted. Try using contrasting colors so they are easy to distinguish from one another.
Take this opportunity to show you’ve done your research and give diversity to your data. For example, you can use a dual axis chart to compare revenue and units sold, price and volume, or rainfall and temperature. If you use similar units of measurement (i.e. monetary values), make sure both y-axes are scaled differently.
Using multiple lines for your dual axis chart is considered acceptable, although it does run the risk of getting cluttered. We suggest you don’t compare more than four variables when creating a dual axis chart.
And remember: Correlation does not always equal causation – other factors may have influenced the data and gone unnoticed.
Have more questions about the appropriate type of chart to use? Check out this article.