A bar chart makes a promise: length stands for size. That promise only holds when the y-axis starts at zero. Slide the baseline up and the data never changes — but the picture does. Every figure on this page is computed from the values actually plotted, so you can watch a real difference of a fraction of a percent turn into a chart that looks like a landslide.
The chart as drawn
The same numbers, zero baseline
Identical data. Nothing here was re-measured — only the axis moved.
Move the baselinethe data stays put
0
as measured
True difference biggest vs smallest—
Difference as drawn what the bars say—
Exaggeration drawn ÷ true—
Vertical zoom vs the zero-based view—
Honestno exaggerationExtreme
Lines are the exception — but not a free pass. A line chart encodes position and change, not length, so a non-zero baseline is often reasonable (nobody plots body temperature from 0 °F). The axis range still controls how steep the change looks, so the rule becomes: read the axis numbers before you read the slope.