How not to be fooled

The axis that does not start at zero

When a bar chart's axis starts above zero, small differences look huge; check the bottom of the axis and compare the real numbers.

By the end of this lesson you can spot a cut-off axis and redraw the bars honestly.

Why it matters

A chart can be built from perfectly true numbers and still give you the wrong idea. The cut-off axis is one of the most common ways it happens, in adverts, news and reports. Once you know to look, you will spot it in two seconds.

The idea in plain words

Imagine two friends standing behind a wall. One is 160 cm tall, the other 166 cm. If the wall is 150 cm high, you only see their heads and shoulders above it: 10 cm against 16 cm. The taller friend looks much taller than he really is. The wall hid the part they have in common.

A cut-off axis is that wall. If a bar chart's axis starts at 50 instead of 0, the bottom of every bar is hidden. Only the tops are left to compare, and small differences look huge.

Cut-off axis (truncated axis): an axis that starts above zero, so the bars are missing their bottom parts and their lengths can no longer be compared fairly.

Same two true numbers. On the right, the axis starts at 50, and Kenya's bar looks more than twice as tall.

See it done

A pupil in Lucía's class finds the right-hand chart online, titled "Kenya is far bigger than Colombia!". Lucía is from Colombia, so the class checks it together.

Data: country-indicators.csv (real), 2024 populations.

Find the bottom of the axis. On the right-hand chart, the lowest number is 50, not 0.
Say what that means. The bottom 50 million of each bar is hidden.
Read the real numbers. Colombia 52.9 million. Kenya 56.4 million.
Compare the numbers, not the bars. 56.4 − 52.9 = 3.5 million more people in Kenya. Kenya has about 7 percent more people than Colombia (56.4 ÷ 52.9 = 1.07, to two decimal places).
See how big the trick is. Above the cut, Kenya's bar shows 56.4 − 50 = 6.4 and Colombia's shows 52.9 − 50 = 2.9. So Kenya's bar looks 6.4 ÷ 2.9 = 2.2 times as tall. The true answer is 1.07 times.
Redraw honestly. Start the axis at 0, as in the left-hand chart. The bars now look almost the same, which is the truth.

Four steps, about ten seconds. Do them for every bar chart you meet.

In Google Sheets: double-click your chart, then Customize, Vertical axis, and type 0 in Min.

Your turn, with help

A bar chart shows two shops' sales: Shop A 95 sales, Shop B 100 sales. The axis starts at 90. How many times taller does Shop B's bar look? How many times bigger are its sales really?

Hint: above the cut, each bar shows its value minus 90.

Show the answer

Above the cut: Shop A shows 95 − 90 = 5, Shop B shows 100 − 90 = 10. So Shop B's bar looks 2 times as tall. Really, 100 ÷ 95 = 1.05 times: only about 5 percent more.

Your turn, alone

A chart shows Mexico (130.9 million) and Japan (123.8 million) with an axis from 120. How many times taller does Mexico's bar look? (Round to one decimal place.)
What is the true answer? (Round to two decimal places.)
In Sheets, make a column chart of Lucía's travel modes (lesson 4.4), then set the vertical axis Min to 3. What happens to the bars? Set it back to 0.
Is a cut-off axis ever acceptable? Think about a line chart of a temperature that only moves between 36 and 38 °C.

Check your answers

Above the cut: 130.9 − 120 = 10.9 and 123.8 − 120 = 3.8. 10.9 ÷ 3.8 = 2.9 times.
130.9 ÷ 123.8 = 1.06 times.
The Walk and Bicycle bars (4 each) shrink to a sliver, and Bus (14) looks far bigger than it is.
For a line chart, yes, sometimes: a line shows change, not size, and a body temperature of 36 to 38 °C would be a flat line on an axis from 0. But it must be clearly labelled. For bar charts, start at 0.

Watch out

Missing axis numbers. If a bar chart shows no axis at all, you cannot check. Be suspicious, and look for the real values.
Small gaps made dramatic. Whenever a chart makes a gap look huge, read the two numbers and work out the real difference.
Bar charts must start at 0. Bar length is the message. Cut the bars, and the message is false.

Say it back

What is a cut-off axis?
What is the first thing to check on any bar chart?
In the Kenya and Colombia example, how many times bigger was Kenya really?

Show the answers

An axis that starts above zero, hiding the bottom of every bar.
Where the axis starts: is it 0?
About 1.07 times (Kenya 56.4 million, Colombia 52.9 million).

What is next

Which country gives off the most carbon dioxide? You might think the answer is easy. It is, until you ask: "per person?" Next lesson.

Data in this lesson: Global Carbon Budget (2025); UN World Population Prospects (2024), processed by Our World in Data. Licensed under CC BY 4.0 (UN data: CC BY 3.0 IGO). Extract and continent labels by DataMedics.