# 4.9: Line Graphs

- Page ID
- 19044

Skills to Develop

- 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.

**Figure \(\PageIndex{1}\): (left) **A bar chart of the percent change in the CPI over time. Each bar represents percent increase for the three months ending at the date indicated. (right) **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.

**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.

**Figure \(\PageIndex{4}\): **A line graph, inappropriately used, depicting the number of people playing different card games on Sunday and Wednesday.