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2.9: Line Graphs

Last updated

Apr 23, 2022
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- 2.8: Bar Charts
- 2.10: Dot Plots

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Learning Objectives

Create and interpret line graphs
Judge whether a line graph would be appropriate for a given data set

A line graph is a bar graph with the tops of the bars represented by points joined by lines (the rest of the bar is suppressed). For example, Figure $\PageIndex{1}$ was presented in the section on bar charts and shows changes in the Consumer Price Index (CPI) over time.

CPI bar chart.jpg — Figure $\PageIndex{2}$ : A line graph of the percent change in the CPI over time. Each point represents percent increase for the three months ending at the date indicated.

CPI line graph.jpg — Figure $\PageIndex{2}$ : A line graph of the percent change in the CPI over time. Each point represents percent increase for the three months ending at the date indicated.

A line graph of these same data is shown in Figure $\PageIndex{2}$ . Although the figures are similar, the line graph emphasizes the change from period to period.

Line graphs are appropriate only when both the $X$ - and $Y$ -axes display ordered (rather than qualitative) variables. Although bar graphs can also be used in this situation, line graphs are generally better at comparing changes over time. Figure $\PageIndex{3}$ , for example, shows percent increases and decreases in five components of the Consumer Price Index (CPI). The figure makes it easy to see that medical costs had a steadier progression than the other components. Although you could create an analogous bar chart, its interpretation would not be as easy.

CPI line graph fig3.jpg — Figure $\PageIndex{3}$ : A line graph of the percent change in five components of the CPI over time.

Let us stress that it is misleading to use a line graph when the $X$ -axis contains merely qualitative variables. Figure $\PageIndex{4}$ inappropriately shows a line graph of the card game data from Yahoo, discussed in the section on qualitative variables. The defect in Figure $\PageIndex{4}$ is that it gives the false impression that the games are naturally ordered in a numerical way.

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