The baseline must start at zero

What it is

A zero baseline for bars makes the length representing an absolute magnitude start from a meaningful zero so screen length remains proportional to value. A truncated origin turns bar-length ratios into differences relative to the truncation point while viewers may still interpret absolute magnitude. The title is a strong default for proportional length encoding, not a rule for every floating interval, delta bar, or measure without a meaningful zero.

Why it happens

When length L = kx, a length ratio equals a value ratio. With an origin b, L = k(x-b), so a screen ratio no longer represents the source-value ratio. Truncation commonly enlarges differences among values near b, but direction and size depend on the values and origin rather than a fixed error. Dot plots primarily encode common-scale position and need not display a length from zero. Lines support position, change, and shape tasks, so their range requires a separate task judgment.

Studying it

Compare zero-baseline bars, truncated bars, dots, and delta charts on the same data. Test absolute value, difference, ratio, and trend judgments while varying truncation, axis markings, labels, and display size. Record systematic bias, error distributions, time, and confidence. Separate whether viewers notice the axis from whether geometry still biases them; do not adopt one arbitrary error percentage as a universal acceptance threshold.

Where it stops holding

Floating bars show intervals, waterfall bars show accumulated change, and delta bars show difference from a reference. Their geometric object is not an absolute value from zero and their endpoints must be explicit. Celsius temperature lacks a meaningful ratio zero, so bar length should not imply “twice as much.” Negative values may extend from zero in the opposite direction, and mixed signs require a visible zero. When tiny differences are central, use dots, deltas, or a contextualized local view.

Applying it

• Lock zero by default for ordinary bars representing absolute magnitude, and require an explicit geometric object and task for exceptions.
• For small differences, prefer dots, delta charts, or direct labels while retaining full-range context and source values.
• Check whether unit and zero support ratio interpretation; avoid absolute bar-length ratios for measures with arbitrary zeros.
• Expose values, units, references, and differences to screen readers rather than relying on length or a broken-axis symbol.

Related

• Same group: U2.02.1 Bar charts suit magnitude comparison across categories · U2.02.3 Order categories by value, not alphabet
• Nearby: U3.01.2 Position encodings tolerate truncation when clearly marked · U1.02.1 Position along a common scale is the most accurate channel
• Search terms: zero baseline · truncated axis · bar chart · ratio distortion