Dual axes and correlation

Manufacturing a relationship.

Why they mislead

Two series on separate scales can be made to track each other by sliding either axis. The apparent relationship is a choice made by whoever drew it.

Spurious correlations between unrelated series are easy to find in any large dataset.
Prefer two stacked charts sharing a time axis.

A scatter plot with a fitted line invites causal reading. Adding the raw points and stating the strength of the relationship keeps the reader honest.