What makes a chart misleading, and how do you spot truncated y-axes, dual axes, and 3D distortion?
Charts mislead when visual area or slope no longer encodes the underlying ratio faithfully. The three most common traps are a truncated y-axis that magnifies trivial differences, dual axes that let the designer set any ratio between scales, and 3D perspective that foreshortens far elements and inflates near ones.
How to think about it
Truncated y-axis
Starting the y-axis at a value other than zero compresses the visual baseline so that small absolute differences appear dramatic. A stock price that moved from 98 to 102 looks like a 400 % surge if the axis runs from 96 to 104. The fix: start bar chart axes at zero. For line charts showing genuine variation around a non-zero baseline (e.g., body temperature 36–38 °C), a non-zero axis is sometimes justified, but label it conspicuously and state absolute values.
Dual axes
Two y-axes let the designer choose the scale of each independently. By stretching or compressing one axis, any two unrelated series can be made to look correlated or divergent. When a presentation shows two lines crossing dramatically on a dual-axis chart, ask: what happens if you re-scale either axis by a factor of two? If the story changes, the axes are doing the work, not the data. Prefer faceted small multiples or normalize both series to an index (e.g., 100 = period 1) instead.
3D charts
3D perspective adds a depth dimension that carries no data. It foreshortens far bars, making them appear shorter, and inflates near bars. Viewers also struggle to read values off a 3D axis because the gridlines are skewed. There is no analytical question that a 3D chart answers better than its 2D equivalent.