Misleading Graphs: Broken Axes and Stretched Pictures

0:00Can a graph of true numbers still mislead you?

0:04A school café sold 50 hot chocolates in week 1 and 55 in week 2.

0:10In graph A, the vertical axis, the scale up the side, starts at 0.

0:17The bars reach 50 and 55. They look close, and they are close.

0:23Graph B starts at 48, which cuts off the bottom of each bar. That is a broken axis.

0:31Above 48, the bars show 50 minus 48, which is 2, and 55 minus 48, which is 7.

0:407 divided by 2 is 3.5, so the week 2 bar looks 3.5 times as tall.

0:48The real change is 55 divided by 50, which is 1.1 times: just 5 more cups.

0:56The common mistake is comparing bar heights without reading the scale.

1:02A picture graph uses pictures for amounts. Say sales doubled, from 50 to 100.

1:10To show 100, it draws the cup twice as tall, so it is twice as wide too.

1:18Its area is 2 times 2, which is 4, so it looks four times as big.

1:24A fair picture graph keeps every cup the same size, one cup for each 50.

1:30Make three checks. First, where does the axis start? If not 0, read the numbers.

1:38Second, are the steps even? Each jump should be the same, not 10 then 40.

1:44Third, what is the source: who collected the data, and could they gain from it?

1:51Read the scale, not just the heights: check the start, the steps and the source.