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Teaching mathematics
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Other types of bar chart

Bar charts can also be used to compare two sets of data in either dual or composite bar charts.

A dual bar chart showing rainfall in London and Bristol.
Figure 19 A dual bar chart
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A dual bar chart showing rainfall in London and Bristol. The horizonal axis shows months from January to May and the vertical axis shows the amount of rainfall in cm. There are two bars associated with each month. A key shows that the blue bars represent London and the orange bars represent Bristol. The heights of the bars are: January 45 (London) 48 (Bristol), February 20, 16, March 31, 33, April 40, 44 and May 14, 11.

Figure 19 A dual bar chart

A dual bar chart shows two sets of data side by side for comparison (Figure 19).

A composite bar chart showing ice cream sales.
Figure 20 A composite bar chart
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A composite bar chart showing ice cream sales. The horizonal axis is labelled with days of the week from Monday to Sunday. The vertical axis shows the frequency of sales. Each composite bar is made up of orange and blue bars which are joined together to make longer bars. A key shows that blue represents beach front sales and orange represents sales in the town centre. The frequencies for each bar are as follows: Monday: total = 41, blue bar = 12 Tuesday: total = 40, blue = 15 Wednesday: total = 30, blue = 11 Thursday: total = 24, blue = 9 Friday: total = 40, blue = 18 Saturday: total = 78, blue = 49 Sunday: total = 72, blue = 43 The orange bars can be calculated from these values.

Figure 20 A composite bar chart

A composite bar chart is useful when the two sets of data are connected, as the frequencies are combined. (Figure 20) For example, when recording the takings from two branches of a shop, a composite bar chart will show the total takings from both branches but also show the breakdown of sales from each shop.

Line graphs

Line graphs are often used to show changes or trends over time. They are produced by plotting a series of points which are joined up by a straight line.

The general shape of a line graph allows some interpretation at first glance. For example, in Figure 21, it is clear that the rainfall was highest on the third day.

A line graph showing rainfall in mm over 9 days.
Figure 21 A line graph
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A line graph showing rainfall in mm over 9 days. The horizontal axis has the title “days” and is labelled from 0 to 9. The vertical axis shows rainfall in mm. straight lines join each data point creating a zig-zag shape. There are data points at the following coordinates: (1,5), (2,2), (3,12), (4,7), (5,8), (6,4), (7,3), (8,2), (9,6)

Figure 21 A line graph

Activity 13 Interpreting a line graph

Timing: Allow 5 minutes
  1. On which day was the rainfall lowest?
  2. On which day did 7 mm of rain fall?

Discussion

  1. Days 2 and 8 both had the lowest rainfall of 2 mm.
  2. A total of 7 mm of rain fell on day 4.