Circle or ‘pie’ charts use pie segments to represent relative proportions of a total measure (ie a part-to-whole relationship). However, because of their shape, the size of these segments is difficult for readers to judge, mentally assign a value to, and compare (Few 2012). The choice of colour can also influence how well readers can compare the size of sections – sometimes a smaller section will appear larger because it is in a strong colour.

Although pie charts have undeniable visual appeal and can be useful in certain situations, many authoritative sources therefore advise not to use them. Use a simple bar graph if accuracy is important (ie bars representing each proportion of the total measure). Ensure that all bars add up to 100% or the total absolute value.

Sometimes, you will need to compare a series of related part-to-whole relationships – for example, the proportion of a total species’ population located in each Australian state or territory, for several species. You probably want the reader to compare the size of populations in the same state, across species. Visually, this is very difficult to do with pie charts. Instead, consider using a stacked bar graph, where the colour and order of like parts (eg states or territories) is identical, and each bar represents a whole group (eg species):