1.2 Back-to-back bar charts
Instead of placing two sets of data on bars side by side as in a dual bar chart, an alternative is to place them together in one line centred around a zero axis.
Here is the same data as shown in Figure 3 only this time presented as a back-to-back bar chart.
Figure 4 Back-to-back bar chart showing monthly rainfall in two locations, Stornoway and CamborneSource: https://www.metoffice.gov.uk/research/climate/maps-and-data/historic-station-dataAs you saw in Week 4, bar charts can be presented with the bars in a vertical or horizontal orientation.
Perhaps you have a preference between the dual or back-to-back bar chart as a representation for two sets of data. The dual bar chart allows direct, side by side comparison of the amount of rainfall per month in the two locations, whereas the back-to-back bar chart gives a better sense of the overall amount of rainfall across the two locations each month whilst also showing the information for the two individual locations.
Some of the earlier representations we have introduced, such as dot plots or stemplots, can also be extended to back-to-back plots when comparing two datasets. You can find a related classroom stemplot Oak National Academy resource here.
ACTIVITY 2
Back-to-back bar charts can be seen as a special case of stacked bar charts. Here are both for the same set of rainfall data you just looked at.
Figure 3 Stacked bar chart showing monthly rainfall in two locations, Stornoway and Camborne Figure 4 Back-to-back bar chart showing monthly rainfall in two locations, Stornoway and CamborneWhat is same and what is different in these two representations?
Complete the table below.
| Same | Different |
Comment
Discussion
Here are some possible points, you may have others.
| Same | Different |
The overall length of the bars is the same amount of rainfall on each graph. The length of the section representing each location is the same amount of rainfall on each graph. The overall number of bars is the same. The bars have gaps between them in both representations. The colours used in the key are the same. |
In the stacked bar chart, all the bars start in the same place (on the horizontal axis in this case). In this example, the bars are horizontal in the back-to-back chart and vertical in the stacked chart (although is possible to produce a horizontal stacked bar chart as well). It may be easier to compare the overall frequencies for the two sets of data in the back-to-back chart, whereas in the stacked chart it is easier to see each dataset as a proportion of the whole bar. Each set of data has its own scale (x-axis) on the back-to-back chart, whereas on the stacked chart there is only one scale (y-axis). |