Misleading scales

Critical judgement of scale is very important when interpreting graphs.

House prices

Glancing at the first bar graph, it appears that house prices have tripled. This is misleading. Looking carefully at the scale, we can see that the scale begins at $79 000.

If we start the scale at zero, as the second bar graph does, then the true picture emerges. Actually, house prices have only increased marginally over the period (2.5%).  

Weight of a puppy

Both of these line graphs have scales that begin at zero.

However, in the first graph, the increments are uneven. This gives the impression that the puppy's greatest increase in weight occurs between month 1 and month 2.

The second graph has a scale with even increments. This graph shows that the weight gain between month 1 and month 2 is lower than that of the other months. This is the opposite interpretation!

There is advice for teaching scales in graphs and an activity for students about scale in graphs.