Other displays for partitions of a total

Two variations of the standard bar chart are often encountered. A stacked bar chart is simply a bar chart in which the bars are stacked on top of each other. It is particularly useful when comparing several sets of values since the stacked bar charts can be drawn side by side.

In a pie chart, a circle is split into segments according to the proportion of data values in each category. The angle for each category is given by the proportion.

Although pie charts seem visually different from the two types of bar chart, they are closely related.

In bar charts, stacked bar charts and pie charts, the area of ink for any category equals the proportion of the total in that category


World cereal production in 2006

The table below shows the total production of cereals (millions of tonnes) in the six continents of the world in 2006.

Region Cereal production
Asia
Europe
Northern America
Central & S America
Africa
Oceania
1102.3       
403.6       
397.5       
192.7       
145.9       
17.2       

The diagram below shows a bar chart of these data.

Drag the slider to the right to stack the bars of the bar chart.


In the diagram below, drag the slider to change the stacked bar chart into a pie chart.


Warning

Stacked bar charts and pie charts should only be used to display values that are partitions of a total. The sum of two values and the total of all values must be meaningful quantities since the graphs imply that the different segments represent proportions of some total.

Infant deaths from abuse

The following example shows data that should not be displayed in a pie chart.

The pie chart below was published in a New Zealand newspaper as part of an article on child abuse.

Since the value from each country is a rate of deaths per 100,000 live births, it is meaningless to add these for different countries — the total cannot be interpreted. A pie chart should therefore not be used.

A bar chart would be a better display of these data. (It would also allow more accurate comparisons between the rates in different countries — it is fairly difficult to compare the areas of different slices above.)