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
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
Infant deaths from abuse
This example shows data that should not be displayed in a pie chart.