20 Reading: Types of Graphs
Three types of graphs are used in this course: line graphs, pie graphs, and bar graphs. Each is discussed below.
Line Graphs
The graphs we’ve discussed so far are called line graphs, because they show a relationship between two variables: one measured on the horizontal axis and the other measured on the vertical axis.
Sometimes it’s useful to show more than one set of data on the same axes. The data in the table, below, is displayed in Figure 1, which shows the relationship between two variables: length and median weight for American baby boys and girls during the first three years of life. (The median means that half of all babies weigh more than this and half weigh less.) The line graph measures length in inches on the horizontal axis and weight in pounds on the vertical axis. For example, point A on the figure shows that a boy who is 28 inches long will have a median weight of about 19 pounds. One line on the graph shows the length-weight relationship for boys, and the other line shows the relationship for girls. This kind of graph is widely used by health-care providers to check whether a child’s physical development is roughly on track.
| Boys from Birth to 36 Months | Girls from Birth to 36 Months | ||
|---|---|---|---|
| Length (inches) | Weight (pounds) | Length (inches) | Weight (pounds) |
| 20.0 | 8.0 | 20.0 | 7.9 |
| 22.0 | 10.5 | 22.0 | 10.5 |
| 24.0 | 13.5 | 24.0 | 13.2 |
| 26.0 | 16.4 | 26.0 | 16.0 |
| 28.0 | 19.0 | 28.0 | 18.8 |
| 30.0 | 21.8 | 30.0 | 21.2 |
| 32.0 | 24.3 | 32.0 | 24.0 |
| 34.0 | 27.0 | 34.0 | 26.2 |
| 36.0 | 29.3 | 36.0 | 28.9 |
| 38.0 | 32.0 | 38.0 | 31.3 |
Not all relationships in economics are linear. Sometimes they are curves. Figure 2, below, presents another example of a line graph, representing the data from the table underneath. In this case, the line graph shows how thin the air becomes when you climb a mountain. The horizontal axis of the figure shows altitude, measured in meters above sea level. The vertical axis measures the density of the air at each altitude. Air density is measured by the weight of the air in a cubic meter of space (that is, a box measuring one meter in height, width, and depth). As the graph shows, air pressure is heaviest at ground level and becomes lighter as you climb. Figure 1 shows that a cubic meter of air at an altitude of 500 meters weighs approximately one kilogram (about 2.2 pounds). However, as the altitude increases, air density decreases. A cubic meter of air at the top of Mount Everest, at about 8,828 meters, would weigh only 0.023 kilograms. The thin air at high altitudes explains why many mountain climbers need to use oxygen tanks as they reach the top of a mountain.
| Altitude (meters) | Air Density (kg/cubic meters) |
|---|---|
| 0 | 1.200 |
| 500 | 1.093 |
| 1,000 | 0.831 |
| 1,500 | 0.678 |
| 2,000 | 0.569 |
| 2,500 | 0.484 |
| 3,000 | 0.415 |
| 3,500 | 0.357 |
| 4,000 | 0.307 |
| 4,500 | 0.231 |
| 5,000 | 0.182 |
| 5,500 | 0.142 |
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