Does a bar graph have to start at zero?

Yes. A bar shows its value by its length, so the value axis must start at zero for the lengths to be comparable. If it starts higher, every bar loses the same chunk off its bottom and the differences look larger than they are.

Can a line graph start above zero?

Often, yes. A line graph shows values by position, not length, so a scale that starts near the data can be honest, as long as it is labelled and the title makes clear what changed. Small changes on a big number, like body temperature, need it.

What is the lie factor?

The lie factor, from Edward Tufte’s 1983 book The Visual Display of Quantitative Information, is the size of an effect as shown in a graphic divided by its size in the data. An honest graph has a lie factor of 1.

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Why a bar graph has to start at zero

A bar shows its number by its length. Cut the bottom off the axis and every bar loses the same chunk, so small differences suddenly look huge.

Honest. School C’s bar is 4% longer than School A’s, as its score is 4% higher.

Misleading. The same numbers: School C now looks three times School A.

Made-up example data.

What happened

In the second graph the axis starts at 90, so School A’s bar shows only the 2 points above 90 and School C’s only the 6. Your eye compares 2 with 6 and sees “three times as much”. The real comparison is 92 with 96: C is about 4% higher.

Edward Tufte called this ratio the lie factor: the size of the effect shown in the graphic divided by the size of the effect in the data. Here the bars show C as 200% bigger than A while the data says 4.3%, a lie factor of about 46. An honest graph scores 1.

How to spot it

Look at the first number on the value axis. On a bar graph it should be 0.
Watch for a zig-zag “break” mark at the bottom of the axis. It admits the cut, but the bars still mislead.
Read the actual values. If two bars look wildly different but the numbers are close, the axis has been cut.

But line graphs don’t always start at zero

That’s right, and it isn’t a contradiction. A line graph shows each value by its position, not by a length from the baseline, so zooming the scale in on the data can be honest, as long as the axis is labelled. Body temperature is the usual example: a fever from 37 to 39 °C is serious, and on an axis starting at zero it would be a flat line. Bars are different: if the length doesn’t start at zero, it isn’t the value any more.

If the differences you care about are tiny compared with the values, a bar graph is the wrong picture. Plot the differences as bars (each school’s score minus the average, say), or use a line or dot chart.

The maker on this site never cuts a bar graph’s axis. The second graph above was drawn specially for this page.

Make an honest bar graph