---
name: Dual axis
slug: dual-axis
category: pattern
status: published
created: 2026-08-21T00:00:00.000Z
modified: 2026-08-21T00:00:00.000Z
definition: Two value axes sharing one plot so two series can be compared, and a
  free knob for manufacturing whatever correlation you like.
aliases:
  - name: secondary axis
  - name: dual y-axis
  - name: two y-axes
  - name: combo chart
tags:
  - dataviz
  - perception
relations:
  contrastWith:
    - truncated-axis
  variantOf: []
  partOf: []
  seeAlso:
    - axis
    - small-multiples
implementations: []
sources:
  - title: "Datawrapper: Why not to use two axes"
    url: https://www.datawrapper.de/blog/dualaxis
demo: inline
exhibit: false
useWhen: someone proposes a second y-axis and the scales question needs asking
---

A second value axis is the standard answer to a real problem: two series that belong in the
same story and share no unit. Revenue is in thousands, complaints are a count, and neither
one drawn against the other's scale says anything. So the plot grows a right-hand axis, one
series is read against each, and the chart appears to have solved it. What it has actually
done is take the single most important thing the reader will do with the picture, compare
the two shapes, and hand the person who drew it a knob that decides the answer.

The specimen above never changes a number. Eight months of revenue, eight months of
complaints, drawn as the same bars and the same line throughout. All that moves is the
domain of the right axis. Set it wide and the complaint line lies low and almost flat under
the bars, and the chart says complaints are a rounding error against a business that is
growing. Set it tight and the identical eight counts climb most of the plot, steeper than
the bars, and the chart says complaints are outrunning revenue. Both pictures are correctly
labelled. Both are drawn from the same file. The crossing point where the line meets the
bars, which is the thing every reader takes away, exists nowhere in the data: it is a
property of the two domains, and whoever chose them chose it.

There are honest uses, and they are narrower than the pattern's popularity suggests. When
the second axis is a fixed transformation of the first, degrees Celsius against degrees
Fahrenheit, a count against that same count as a percentage of a known total, there is no
free knob at all, because the mapping is arithmetic rather than taste. The same goes for a
second axis whose domain the subject matter already fixes, such as a rainfall scale a
meteorological convention pins. Where the two units are genuinely unrelated, the usual
repairs are to index both series to their own starting value and plot them on one axis in
percent, or to stop overlaying them at all and use
[small multiples](/small-multiples), which keeps the comparison honest by refusing to draw
it as a single shape. [Datawrapper](https://www.datawrapper.de/blog/dualaxis) makes the
stronger version of the argument, which is that a reader cannot tell a real crossing from a
manufactured one, so the chart cannot be trusted even when it is right.

The neighbours divide by which part of the drawing does the lying. A
[truncated axis](/truncated-axis) lies about magnitude, moving one baseline so a difference
looks bigger than it is; a dual axis lies about correlation, handing each series its own
free knob so two shapes can be made to agree or disagree at will. [Axis](/axis) is the
component both are choices about, the drawn line with its ticks and its labels, and
[chart](/chart) is the whole drawing they serve. When the domains are picked to win an
argument the numbers do not support, the word stops being a charting term and becomes a
[dark pattern](/dark-pattern): the evidence is all present, in small type, at the edge
nobody reads.