Crime taxonomy · Axes and scales

Dual-axis distortion

Two series on two independent axes, scaled to tell whatever story you like.

What it looks like

Two lines share a chart but each has its own y-axis. Because each axis range is chosen freely, the lines can be made to overlap, cross or diverge at will.

Example

DUAL-AXIS ABUSE

FIXED

Why it is bad

• Any two rising series can be made to look perfectly correlated by stretching one axis.
• Where the lines cross means nothing, yet readers treat it as a turning point.
• It invites a causal reading ("A drives B") that the data never tested.

How to fix it

1. Index both series to a common starting point (e.g. 2015 = 100) and use a single axis.
2. Or draw two charts side by side, each with its own honest axis.
3. Or plot one against the other in a scatter plot if the relationship is the point.