SQL, data modeling, data quality, EDA, visualization · insight-0069
Start bar axes at zero, and switch to dots or lines when you need to show small differences
- Evidence type
- documented fact
- Difficulty
- easy
- Roles
- Data analyst, Data scientist
- Industries
- cross-industry
Summary: situation, action, reason, boundary
Situation
A bar chart of route load factors starts its axis at 75% so the differences look clearer.
Action
Keep bars anchored at zero. If differences are small relative to the level, plot points or a line on a zoomed axis, or plot the differences themselves (versus target, versus last year) as bars from zero.
Reason
Readers judge bars and shaded areas by their length or area, so the visible ink must be proportional to the value.
Boundary
Bar charts, area charts and filled line charts.
Symptom or risk
Bar lengths no longer represent values, so a 2-point gap looks like a doubling and readers draw the wrong conclusion.
Exceptions
Ratios on a log scale should have bars that start at 1, not 0; dot plots and lines encode by position and can use a non-zero axis.
Trade-offs
Zero-based bars can make meaningful small changes look flat; choose a different encoding instead of truncating.
Pitfalls
Shading the area under a line that starts at a non-zero baseline, which recreates the bar problem.
Lessons and concepts
Claims and their sources
Bars on a linear scale should start at zero (principle of proportional ink); differences can be shown as bars from zero, and bars for ratios on a log scale should start at 1.