2.9 Line Graphs

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 1 was presented in the section on bar charts and shows changes in the Consumer Price Index (CPI) over time.

Bar chart of CPI percent increase for July 2000 (3.8), October 2000 (2.8), January 2001 (4.2), and April 2001 (2.5). — Figure 1. 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.

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

Line graph of the same CPI percent increases connected across the four quarters: 3.8, 2.8, 4.2, 2.5 — the rises and falls are easier to see than in the bar version. — Figure 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.

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 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.

Line graph of CPI percent increase for five components across four quarters (July 2000 to April 2001): Housing spikes to 6.7 in January 2001 then drops to 2.1; Medical Care rises steadily from about 4.2 to 5.3; Food and Beverage dips to 2.4 then recovers to 3.6; Recreation falls to 0.8 before climbing to 3.5; Transportation drops to 0 in October 2000, rises to 2.3, then ends at 1.6. — Figure 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 4 inappropriately shows a line graph of the card game data from Yahoo, discussed in the section on qualitative variables. The defect in Figure 4 is that it gives the false impression that the games are naturally ordered in a numerical way.

Line graph incorrectly connecting the ten card games on the x-axis: number of players on Wednesday and Sunday drawn as two lines from Poker (about 700) up to Spades (about 7,100). The categories have no order, so the connecting lines are meaningless — an example of misusing a line graph. — Figure 4. A line graph, inappropriately used, depicting the number of people playing different card games on Sunday and Wednesday.

Note that the graphs on this page were not created in R. However, the R code shown here produces a very similar graph.

# Figure 3

food=c(4.1,2.4,2.6,3.6) housing=c(4.9,4.3,6.7,2.1) medical = c(4.2,4.5,4.8,5.3) rec = c(3.3,0.8,1.2,3.5) tran = c(4.8,0,2.3,1.6) plot(housing, type="o", xaxt="none",col="purple", xlab="Date", ylab="CPI % Increase",ylim=c(0,7)) lines(food,type="o",col="blue") lines(medical,type="o",col="green") lines(rec,type="o",col="red") lines(tran,type="o",col="black") legend("topleft", legend=c("housing","food","medical","recreation","transportation"), col=c("blue","violet","green","red","black"), lty=1,lwd=2) axis(1, at=1:4, lab=c("July 2000", "October 2000", "January 2001","April 2001"))

Adapted from Online Statistics Education: A Multimedia Course of Study (onlinestatbook.com), Project Leader: David M. Lane, Rice University. Developed with NSF support. The original work is in the public domain; it is cited here at the authors' request. Changes were made: reformatted as an accessible XYZ web edition with native MathML. License: Public-Domain.