Read both axes before interpreting any relationship between the lines.

Dual-axis charts can visually exaggerate or hide relationships because the two series use different scales.

Revenue might rise from 100 to 120 while margin moves from 20% to 21%, yet the chart can make both changes look equally dramatic.

State the units and scale for each series, then compare the actual changes before drawing a conclusion. If useful, this guide on case interview insight generation gives more detail.

Dual-axis charts are sneaky. I always read the scales first.