In Figure 1 you can see that the number of police officers decreased from 1993 to 1996, but started increasing again in 1996. The graph also makes it easy to compare or contrast the number of police officers for any combination of years. For example, in 2001 there were nine more police officers than in 1998.
The double (or group) vertical bar graph is another effective means of comparing sets of data about the same places or items. This type of vertical bar graph gives two or more pieces of information for each item on the x-axis instead of just one as in Figure 1. This allows you to make direct comparisons on the same graph by age group, sex, race, or anything else you wish to compare. However, if a group vertical bar graph has too many series of data, the graph becomes cluttered and it can be confusing to read.
Figure 2, a double vertical bar graph, compares two series of data: the numbers of boys and girls using the Internet at Redwood Secondary School from 1995 to 2002. One bar represents the number of boys who use the Internet and the other bar represents the girls.