Prefer facets over dual axes

What it is

What dual axes try to achieve — two differently-unit-ed series read together — has a far more honest implementation: stacked facets. Two panels share the horizontal (time) axis, each with its own y-axis, axis and line paired one-to-one with unambiguous colour; readers read each trend against its own axis, and the vertical stacking naturally suggests "two things under one timeline" without suggesting "two comparable lines." Faceting spends layout (one more panel of height) to buy out the dual axis's arbitrariness (first leaf) and spurious-correlation risk (second leaf) entirely — each panel's scale is independently declared, and no cross-line geometry exists.

Why it happens

Faceting is honest because it structurally forbids cross-line comparison: the two lines live in different plot regions, and co-variation perception has no trigger (intersections and spacing cues do not physically exist). When readers genuinely need the relationship, the faceted layout nudges them toward the right tool (the scatterplot), whereas the dual-axis layout holds the same reader inside the illusion with "looks comparable." Stacked panels also carry an attention advantage: vertical adjacency makes time-aligned reading smooth (one downward glance at the same moment), inheriting the mature price-volume convention — charts of that class evolved from dual axes to split panels, an industry voting with its feet.

Where it stops holding

The faceted approach carries the facet family's own constraints: each y-axis must be explicitly valued (or it regresses to "hidden scale switching"), with aligned reference lines to aid time registration where needed. If the question is literally "are the two series related," facets are not the destination either — go to the scatterplot (time colour-encoded); facets answer only "how does each vary." In severely space-constrained settings (one-row mini reports), "two tightly stacked small charts" works — still faceting in essence, not dual axes.

Applying it

• Disable the dual-axis default at the template level; two series with different units route straight to stacked facets.
• Facet spec: shared horizontal ticks, each panel's y-axis start and end explicitly labelled, axis colour bound to line colour, a narrow gutter between rows.
• Verification: show dual-axis and faceted versions to the same reader group asking "what is the relationship between the two series?"; the faceted version should yield markedly more "separate descriptions + relationship unverified" answers — the excess is the volume of illusion the dual axis fabricated.

Related

• Same group: U3.04.1 On dual axes, relative placement is an arbitrary choice · U3.04.2 Dual axes can fabricate nonexistent correlation
• Nearby: U2.15.1 Small multiples replace overlay with repeated like-for-like panels · U2.05.1 Scatterplots suit the relation between two continuous variables
• Search terms: faceted layout · dual axis alternative · stacked panels · shared time axis