Dual Y-Axes Only When Causally Linked.

A dual Y-axis can read cleanly when the two metrics are causally related and on different units (temperature vs. ice cream sales, ad spend vs. signups). It misleads when the axes are scaled to make unrelated lines appear to track each other - any pair of trending series can be made to look correlated with the right scaling. Default to two stacked charts or an indexed-to-100 view when in doubt.

Few, 2012; Munzner, 2014 · Visualization Analysis & Design