How to make a dual axis line chart
When to use a dual axis chart
Dual axis charts are a controversial topic. Though they're not uncommon, they don't always help your audience understand certain information. Especially if it's just a question of trying to display more information, there are often better ways to display your data.
The only time you should use a dual axis chart is to show correlation. In other words: Only use dual-axis charts to show when one data series moves in or out of sync with another data series. The reason that a second axis is required is that the measurement units for the two data series are different. If the measurement unit was the same, a simple line graph would be used.
How to make a dual axis chart in Flourish
To add a dual axis to a line chart, select "Dual axis: lines & lines" from the drop-down menu.