On dual axes, relative placement is an arbitrary choice

What it is

Dual y-axes draw two dimensionally different series into one plot region, the left axis governing one series, the right the other. A structural fact usually goes unnoticed: the relative geometry of the two lines is decided entirely by axis parameters. The left axis's start and end and the right's start and end are four free parameters; adjusting them makes the lines overlap, separate, cross, or run parallel — without a single datum changing. When readers see "two lines dancing together," they see the product of those four parameters, not a data relationship; this is the root of the dual-axis problem, and the next leaf's fabricated correlation grows from it.

Why it happens

In a single-axis chart, the relative position of two lines is uniquely fixed by the data (higher is higher under one scale). Dual axes dismantle that uniqueness: each line maps to its own axis, and cross-axis geometry has no definition — "line A sits above line B" corresponds to no data proposition, only to some setting of the axis parameters. Those settings (starts, ends, occasionally scale transforms) are numbers filled in by hand in charting tools, with no objective criterion; the same data under two parameter sets can produce visually opposite "relationships." Four continuous degrees of freedom mean the tunable visual relationships form nearly a continuum.

Where it stops holding

Not every dual-axis use counterfeits a relationship. The legitimate case is "read each against its own": the lines share a time axis only to save space, readers consult each axis separately for each trend, and no cross-line comparison is made — which demands explicit visual assignment (axis colours matched to their lines, a caption saying "axes independent, do not compare across"). Technical-indicator layouts (price over volume, split top and bottom) have already evolved into quasi-faceted stacked panels, in effect conceding this leaf's principle. The true red line is inviting cross-line comparison: lines drawn close, sharing a colour family, without warning — that is the next leaf.

Applying it

• Review question: what is this dual-axis chart's question? If the question involves "the relationship between the two lines," it fails outright — dual axes cannot answer relationship questions.
• Where dual axes stay, enforce four items: axis ticks tinted to their lines' colours, axis parameters archived with the chart, a caption declaring "independent axes," and no visual closeness of the two lines.
• Verification: re-render the same data under two axis-parameter sets and compare readers' "relationship conclusions"; conclusions flipping with the parameters demonstrates the arbitrariness.

Related

• Same group: U3.04.2 Dual axes can fabricate nonexistent correlation · U3.04.3 Prefer facets over dual axes
• Nearby: U3.01.3 Axis truncation is the most common misleading device · U2.17.1 A mismatched chart forces readers to compute the answer mentally
• Search terms: dual axis · secondary axis · scale arbitrariness · compositional fallacy