A broken axis removes a value interval from the middle of the scale

What it is

A broken axis is not a truncated axis: truncation cuts the end (axis not starting at zero); a break cuts the middle — an interval of the scale is excavated and the two sides stitched together, usually flagged by a wavy line or double slash "there is a gap here." The motive is almost always an outlier: one value ten times the rest, where a linear axis either crushes the majority or bursts the page — the break looks like having both, majority filling the frame with the outlier still in. The bill comes due from the next leaf: the geometry at the seam is already destroyed.

Why it happens

The break's essence is two different scales sharing one axis line: below the gap each pixel means a units, above it b units (a ≠ b), while the axis looks continuous. Readers interpolate and judge slopes under the lifelong assumption that the axis is uniform, and every inference crossing the break silently mixes two conversions. The wavy marker itself carries no magnitude — it says "hole here" but not how deep, and the gap's drawn width has no correspondence to the deleted interval, which makes the break harder to self-correct than end truncation: truncation at least admits an amplification factor from the start tick, while the two sides of a break demand separate reconstructions.

Where it stops holding

This leaf sets only the definition and motive; distance corruption, marker failure, length-encoding failure, and alternatives each occupy their own leaf in the group. One boundary can be stated up front: the broken axis has no "legitimate home turf" — a few scientific conventions (spectroscopy and similar) aside, its gain (outliers on-screen) always arrives with the group's costs, so the default stance is don't; use the fifth leaf's alternatives.

Applying it

• Detection drill: at review, scan the axis for wavy or double-slash marks first; any hit triggers the group's remaining checks (marker visibility, length encodings crossing the break).
• If an established convention forces a break: make the gap mark exaggerated (several times the data stroke width), label the value interval on both sides of the break, and note "axis interrupted at X–Y" in the caption.
• Verification: ask readers to report "where is the axis missing a stretch, and how much"; failure means the marker has failed — treat as unmarked.

Related

• Same group: U3.02.2 Across the break, equal distances no longer mean equal values · U3.02.3 Break markers are often too small to notice, equal to unmarked · U3.02.4 Length encodings crossing the break lose all comparability · U3.02.5 Facets or log scales usually replace the broken axis
• Nearby: U3.01.3 Axis truncation is the most common misleading device · U3.03.1 Log scales suit data spanning orders of magnitude
• Search terms: broken axis · axis break · scale gap · outlier axis