Charts that fool you

The idea

A fair chart starts a bar axis at zero, labels its scale clearly, and leaves nothing important out.

Same quiz, different bars?

Three sections took the same quiz. Section 7C's bar looks about five times as tall as 7A's. How much better did 7C do?

• About five times better
• Only a little better
• I cannot tell from the bars alone

Now start at zero

Here are the same marks again. This time the bars start at 0. Tap the bar that answers each question, and notice how tall the bars look.

Where the axis starts

In the first chart the axis started at 90, so a gap of 4 marks looked huge. Starting at 0, the same bars are almost level: 95 against 91.

A bar's length stands for its value, so a bar chart should start at zero. Two more checks: read the labels and scale, and ask what has been left out.

A fair chart starts its bar axis at zero, labels its scale clearly, and leaves nothing important out.

A chart that gives a wrong picture, even from true numbers, is misleading.

Notes

A fair chart starts its bar axis at zero, labels its scale clearly, and leaves nothing important out.

Before you trust a chart, check where the axis starts, what the labels say, and what is missing.

Check yourself

A poster says "Chess club is booming!" above this chart of members each year. What makes it misleading?

A shop shows only its three best months, under the words "Our sales keep growing!" What should you ask?

Priya's section averaged 82 marks and Ishaan's averaged 80. She is drawing a bar chart for the notice board. What should she do?

• The chart is fine, because the bars do get taller each year. The bars do grow, but how big the growth looks depends on where the axis starts. Start at 0 and the growth looks much smaller.
• The axis starts at 16, so small gaps look big — correct. Yes. The small zigzag warns that the axis skips ahead. The 2025 bar looks twice as tall as 2023's, but 24 against 20 is only four more members.
• The years are in the wrong order along the bottom. Years run left to right in order, which is right for change over time. The order is not the problem.
• The chart is fine, since every bar shown is a true number. Each number can be true and the chart can still mislead. Leaving out the slow months hides part of the story.
• Why is it not a pie chart?. The chart type is not the problem. Months in order suit bars or a line. The problem is the nine months left out.
• Why are the other nine months missing? — correct. Yes. Always ask what has been left out. A fair chart shows the whole year, the good months and the bad.
• Start the axis at 0 and write the marks on the bars — correct. Yes. Everyone can see the true gap and check the numbers. An honest chart earns trust.
• Start the axis at 79 so her bar looks much taller. That would turn a gap of 2 marks into a giant. Readers would trust the picture and be fooled.
• Leave out Ishaan's section so hers is the only bar. Leaving a section out hides what people want to compare. A fair chart shows everyone.