Bar charts for quantities
Bar charts are most commonly used to show frequencies for discrete or categorical data.
However it is also acceptable to use a bar chart to display any quantity data. (Quantity data are 'amounts' of something and are always positive. Since it is meaningful to say that one quantity is double another, quantity data are also called ratio variables.)
A bar chart can therefore be used to show how a quantity changes over time (a kind of time series plot) or to show how a total quantity is split between categories.
New Zealand wine production
The bar chart below shows how the area in New Zealand used for vineyards changed between 1962 and 2013. (Area is a quantity — doubling the area is a meaningful concept.)
Select Production from the pop-up menu to see how wine production changed over this period. In contrast to the steady increase in vineyard area, wine production has fluctuated markedly since 1980 and has levelled off.
Another interesting measurement for producers is the ratio of production to area — the production per acre. Select Production per hectare from the pop-up menu to see how this has changed. Production per hectare has steadily dropped since 1970.
Possible explanations are...
Further information is required to assess these explanations and fully understand this pattern.
Select the option Time Series from the pop-up menu on the left. Since the data were recorded each year, time series plots can also be used to display them.
Pie charts for quantities
Pie charts can also be used to display quantity data, but there is an additional requirement that must be satisfied before a pie chart is used. The total of all the data that are displayed must itself be meaningful.
It is unfortunately common for pie charts to be used in situations where the total is not a meaningful quantity. Make sure that you recognise such misleading pie charts and do not draw them yourself.
World crude oil production
The pie chart below shows the source of all crude oil produced in 2013.
This pie chart is not based on categorical data (a list of categorical measurements from individuals), but shows how a continuous total (the total world oil production) is split into categories.
The following example shows data that should not be displayed in a pie chart.
Infant deaths from abuse
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.)