Pie charts are not enemies, but we should accept that our brain is poor at comparing angles.
Humans are much better at decoding numbers which presented in the form of length or position than they in angles or areas. Pie charts invite viewers to try anyways, which increases cognitive load.
Edward Tufte writes (1983) – ’A table is nearly always better than a dumb pie chart.’
William Cleveland is one of the world’s foremost authorities how information is extracted from a graph(1996) – ‘Pie charts have several perceptual problems. Pie charts do not provide efficient detection of geometric objects that convey information about differences in values.’
Naomi Robbins writes: We make angle judgments when we read a pie chart, but we don’t judge angles very well. These judgments are biased; we underestimate acute angles (angles less than 90°) and overestimate obtuse angles (angles greater than 90°). Also, angles with horizontal bisectors (when the line dividing the angle in two is horizontal) appear larger than angles with vertical bisectors.
Pie charts only make it easy to judge the magnitude of a slice when it is close to 0%, 25%, 50%, 75%, or 100%.
They are really useful when:
- Comparing a small number of categories (2-3 different data points).
- Emphasizing percentage of whole relationships.
Variants of pie chart
The previous statements are true for- the pie’s brother chart type – donut chart. We criticized the pie charts due to the angle of each piece of the pie. The donut type has a cut-out center so we don’t have to struggle with the angles problem. Instead, we get the new challenge to estimate the values of the arc lengths. Unfortunately, we can’t easily understand and see data displayed in this form.
Walter Crosby Eells conducted an experiment about how we judge the pie charts: He found in his study that we judge pies by arc length, area or central angle and just a few people by chord length.
If you are interested in this topic I really recommended his study: The Relative Merits of Circles and Bars for Representing Component Parts.
What other chart types we can use to show the proportion.
Simple bar chart or Stacked bar chart
Definitely, the best alternative for a pie chart/ donut chart is a simple bar graph because in that case we only have to compare one dimension, length with more clarity and less cutter.
Ben Shneiderman, the founder of the treemap, which shows the hierarchical data in areas of rectangles.
Nowadays I see more and more square pies, also known as waffle charts. Since it does not involve angles, it’s easier for the reader to compare data accurately than in pie charts.
The waffle chart’s sister is Dot Matrix Chart. It gives a quick overview of the distribution and proportion of the categories.
Stacked are chart
It is useful if you want to show the changes over time for many variables
In the “Women and equality” report, I used the waffle chart method to show the gender proportion in these half circles.
See Andys tutorial about how you can build easily waffle matrixes. https://www.youtube.com/watch?v=wRpBkl6-uhU
You can see the report here: http://Women and equality in national parliaments