Pie chart from Playfair's Statistical Breviary
This anniversary is an opportunity to have a closer look at this seemingly mundane form of visualisation, which was nevertheless the beginning of many visualisations using polar coordinate systems. Despite its long history, the visualisation still provokes discussions among its advocates and opponents.[2]
The simple idea to show the relationship between a part and the whole was easy to grasp. No wonder followers came quickly. It is possible that when Charles Joseph Minard (the one who visualised Napoleon's march on Moscow and designed the map of French wine export) used the pie chart in 1858, he had those features in mind. This is how Ch. J. Minard presented the volumes of cattle transported from individual regions of France to Paris (chart courtesy of Wikipedia). Apart from cattle breeds within each chart, there is an additional feature there: the size of each pie is in proportion to the volume of animals transported.
What is the weakness of this form of data presentation? Unfortunately, it is its strength: the ease with which a part relates to the whole. While it worked well for W. Playfair and Ch. J. Minard, it rarely works unless:
- there are few slices of the pie (‘few’ meaning 3 rather than 8);
- the values of the slices are close to 0% or 100% (the differences are substantial and easy to spot quickly);
- the values of the slices are close to quarters and halves (they are easy to relate to the material reality);
- the arrangement of the slices makes it easy to read the chart (for example the beginning of each slice is clockwise, at 12, 3, 6, or 9);
In other circumstances, the strength of the pie chart becomes its weakness because the human eye fails to interpret acute and obtuse angles. This is where misunderstanding or even manipulation can sneak in.
Should we get rid of the pie chart from the menu then? Of course not. There is no rule of the thumb. Just do not overuse it to prevent indigestion.
The chart below shows costs of a birthday party. The thorough organiser wanted to summarise the expenses this way.
This chart is a good opening for data exploration. Even without percentages, you can clearly see that food and beverages were the largest expense with the venue and service fees coming in second. At the same time, venue and service fees together with the band covered over half of the budget. An image of the event already starts forming.
To sum up, you should not replace other types of visualisations with the pie chart. Also, do not expect the pie chart to effectively convey every piece of information just as you would not expect to be full after a starter; it should leave you craving for more.
[1] If you want to learn more about this colourful persona, start with Wikipedia
[2] One of the most interesting discussions on the practical value of the pie chart is an article by Stephen Few Save the Pies for Dessert (the title alone divulges the author's view but the arguments are worth it)"