Truncated axis

A value axis does not have to start at zero, and most of the long-running argument about misleading charts is really an argument about when it does. Truncating means beginning the scale just under the smallest number in the data, which spends the whole plot on the range those numbers actually occupy. Every difference gets bigger. Whether that reads as clarification or as exaggeration depends entirely on what the marks encode, which is why “always start at zero” is a rule that is right half the time and confidently wrong the other half.

Bars encode value as length measured from a baseline, so the baseline is part of the encoding rather than a framing choice around it. Claus Wilke states the constraint as proportional ink: the ink a mark spends has to be proportional to the value it stands for, and a bar cut off underneath is spending its length on a quantity nothing in the chart names. The specimen above runs the same four quarters both ways. Handset share moves from 46.8 to 48.4 percent, a rise of 3.4 percent, and with the axis cut at 46.4 the last bar is drawn five times the height of the first. Nothing in the drawing is false. Every bar ends at the correct tick, and the tick is labelled. The reader is simply being invited to compare lengths that have stopped meaning what lengths mean.

Lines and dots are the other case, and they are the reason the zero rule cannot be applied blind. A line encodes value as position, read against the labelled scale rather than against the bottom of the frame, so a chart of body temperature, of a share price, or of pH has no business starting at zero: the plot would be a flat band with every reading crushed into a few pixels. Truncation there is not a trick, it is what makes the chart legible at all. The dishonesty is never the cut. It is cutting silently, with a scale small enough to go unread and a shape loud enough to be remembered, which is what the axis break glyph (the little zigzag through the baseline) exists to prevent. Calling Bullshit collects the cases where that signal was left off on purpose.

Three neighbours separate this cleanly. Axis is the component a truncation is a choice about: the axis is the drawn line, its ticks, and its labels, while truncation is a decision about the domain those labels report, so the same axis component draws both charts in the specimen. Chart is the whole drawing the axis serves. And when the cut is made deliberately, to win an argument the numbers do not support, the word for it stops being a charting term: that is a dark pattern working by misdirection, sending attention to a shape while the evidence sits in small grey type at the left edge.