Drill Down Combo PRO supports two independent y-axes on the same chart. This feature can be used to accurately display multiple series on the same chart in cases where their value ranges vastly differ (for example, revenue in millions and employee count in hundreds).

Enabling this toggle will create a secondary value axis, and you can then go to the Series X Customization section of each individual series to assign them to your desired y-axis with the Value Axis setting. 

Once the secondary y-axis is enabled, additional customization options will be revealed which you can use to visually distinguish the two axes from each other. These options behave exactly like Primary Y-Axis Configuration settings, apart from the Align to Primary Axis toggle, which is exclusive to the secondary y-axis.