The Truncated Axis
In the dock: A quarterly slide that needed the number to look big
The crime
Two bars, a 2% gap. By starting the axis at 96 instead of 0, one bar now towers three times over the other. Same data, different lie.
The fix
Bar charts start at zero. If the real difference is too small to see, that's information — switch to a dot or line chart and label the range.
The law it breaks
Tufte's Lie Factor — the ratio of the effect shown to the effect in the data. A bar's length is its claim, so a bar chart only tells the truth from zero.
A bar says its whole length — that’s the contract the reader signs the instant they look. Length is value. So the moment the axis is cut off at 96 instead of 0, the contract breaks: a 2% difference stretches into a bar that looms three times taller than its neighbour. The numbers never changed; the drawing lied about them. Tufte gave it a number — the lie factor — and this chart’s is enormous.
The honest caveat, so this is a receipt and not a rule chanted from memory: truncated axes aren’t always a crime. A line chart of already-large values (a stock sitting at $412–$418 all week) can start above zero, because a line encodes position, not length — a flat line honestly reads as “not much happened.” The sin is specific to bars, whose length is the entire message.
So: bars start at zero. A difference too small to see that way isn’t a problem to paper over — it’s the finding.