Explain the concept of Dual Axis.

A dual axis puts two measures on the same view with two independent axes, one on each side, layered over one another rather than side by side.

You create it by placing the second measure on the opposite side of the view, or by right-clicking the second pill and choosing Dual Axis. Each measure keeps its own Marks card, so one can be bars and the other a line.

It is genuinely useful for a measure against its target, or for two quantities in different units over the same period.

The risk is that independent scales let you imply any relationship you like. If both measures share a unit, right-click the secondary axis and Synchronise it — otherwise the comparison is misleading. That is the follow-up interviewers usually ask.