Turn dual axes into indexed lines
Choose an indexed-line repair when the business question is relative movement over time.
A product team needs to compare usage and cost without implying that the raw units are comparable. Before: two units, two scales, one misleading visual relationship. After: one indexed scale compares relative movement without fake equality. Both series start from a common baseline, so the viewer compares percent movement rather than raw incompatible units. The second y-axis is removed, eliminating the ability to manufacture crossings through scale choice. The chart title should name the narrower claim: relative movement over time, not causation. If the claim becomes relationship strength, the next chart should be a scatterplot or model, not another time-series…
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