The dual-axis is used to visualize two different measures in two different chart types. A date column and two measures are necessary to build a dual-axis chart.

In Tableau, a dual axis refers to the capability of having two separate axes (both vertical and horizontal) on the same chart. This feature allows you to plot two different measures with different scales on a single chart, facilitating a more effective comparison between them.

By using dual axes, you can visualize and analyze the relationships between two measures that might have different units or scales. This can be especially useful when you want to compare data that would otherwise be challenging to interpret on a single axis.