Vertical or horizontal bars?

The book has a table of the tallest mountain on each continent: Everest (Asia) 8848 m, Aconcagua (South America) 6962 m, Denali (North America) 6194 m, Kilimanjaro (Africa) 5895 m, Elbrus (Europe) 5642 m, and so on. A long table of numbers is hard to compare quickly, so it becomes a bar graph.

First the heights are drawn as horizontal bars, lying sideways. That works, but a mountain's height goes up from the ground. So the picture is turned round, and the bars now grow upward, just like mountains. A bar graph with vertical bars is also called a column graph. (Columns are the pillars that hold up a roof.)

The book's advice:

• Things measured upwards from the ground (heights of mountains, buildings or people) look most natural as vertical bars.
• Lengths that lie along the ground (the distance between two places, the length of a river) look most natural as horizontal bars.

Both kinds show the same numbers. Choosing the right direction just makes the graph easier to understand at a glance.