Ever since the dawn of history, some go-to techniques have wormed themselves into mankind’s habits, despite having no business being there. That clichéd opening for an essay is one example.
Another is pie charts.
Pie charts are an extraordinarily simple idea: to show the different sizes of things, make a graph. Let them take up proportionally different parts of a shape, and since round circles are welcoming shapes that analog clock users are already familiar with, shape it like that. Color code everything, and you’re good to go–instant data visualization.
Pie charts are overrated, however–they are generally ineffective at their purpose, and knowing this should make you look more skeptically at anyone who presents a pie chart to (ostensibly) communicate. Even the evidence that pie charts have begun their inevitable long slide into obscurity, after their early 2000’s heyday, does not mean that they don’t deserve a fulsome autopsy once and for all.
The basic purpose of a pie chart is to compare different amounts. The basic problem with pie charts is that circular segments are not easily compared by the human eye. Consider this apt example from Wikipedia, brought to our attention by Business Insider:
Good luck digging up a protractor to calculate which colors are pluralities in these similar-ish charts. If a chart that is designed to show you how things compare requires numbers everywhere to tell you what everything means, it isn’t doing its job.
Behold the same data:
At a quick glance, the data is right there for your eyes to take in. Knowing that the same scale applies for all five bars is of course essential (and naturally taken for granted), but it’s also an afterthought to the comparison.
This is considering pie charts in their best light and their most essential form. By now, I expect that some of you are expecting to hear about the monstrosity that is “3D pie charts”–a backwards innovation that grasps at the prestige of more sophisticated design while it lets content obfuscation in at the front door.
Wherever someone’s visualization lets a third dimension peek into the image, they are giving that chosen slice extra real estate on what remains a 2D image. Here’s that chicanery (helpfully pointed out by geckoboard) on full display at MacWorld a few years ago:
Green’s slice of the pie is 19.5%, but since the perspective tilts the pie to show off its “3D” quality, green takes up more of the image you’re actually looking at–you see the edge of the pie, while perspective causes that part to seem closer to you and more spread out as the background slices of purple and blue start to flatten. (Imagine tilting your desk forward until you’re almost confronted with the edge–the part furthest from you is basically a line now, while the small part close to your face can still be seen in some detail.) To top it off, the bigger, green slice is Apple’s in the chart–what a coincidence!
Nevertheless, one point often made in defense of pie charts (where they have their defenders) is that fair, two-dimensional pie charts are intuitive if you don’t split it up too finely. O’Reilly makes the point with this example:
If the pie chart works well in this scenario, does it do the job better than a comparable plot, however?
Behold the bar plot:
The bar plot communicates as well, if not better. For one thing, it gives more immediate visible comparisons between any pair of elements present, not just adjacent ones or anything paired with the biggest. It also gives a better idea of relative sizes–boxes may not be as visually appealing as slices of a round circle, but they are more easily compared than the variously oriented triangular approximations that are segments of a pie chart.
All that said, there are two, perhaps three, worthwhile uses for a pie chart.
Pie charts, people have pointed out, are properly geared towards demonstrating proportions of a whole. (In this way they are different from donut charts, which demonstrate proportions around a hole.) Thus, if you have a meaningful whole and are trying to show what it is comprised of, a pie chart might be good at getting across the idea of a single thing’s breakdown. Proposed (and actualized) budgets, for example, are apt candidates here, like this one from National Priorities:
(Don’t ask me what the slice labeled “Government” means on this pie chart, because I couldn’t tell you.)
Bar charts, on the other hand, are arbitrary comparisons of items in a list. The list could be exhaustive of some category (“crimes committed by months of the year”) or a haphazard selection, as below:
Are the countries depicted the top seven in gold reserves? The chart seems to suggest that–but it doesn’t affirm as much. (Incidentally, this is one blind spot bar charts have where you are liable to be led astray–whenever you are presented with a bar chart, it is necessary to know the exact selection principle and what may have been omitted.)
Another specific use case for pie charts is when you are trying to compare one element to a vast field of other, much smaller members. Consider, for example, countries’ GDPs:
This would be better still, naturally, if the slices continued ever smaller. Even so, this pie chart of the countries of the world has a well-defined list or whole to demonstrate proportions of, in a way that a bar graph could only clumsily illustrate, since you would need so many bars. A simple graph of dots or crosses on a number line might get the point across, but not as dramatically or colorfully.
One last note in favor of pie charts: When comparing hypothetical coalitions or combinations, grouping wedges in a circle is easier than mentally relocating and stacking bars on top of each other. For instance, considering party composition of a parliament, here are the results of the European Parliament’s elections in 2004:
Trying to mentally piece together a majority coalition is easier here than it would be with a bar graph.
Except for those three specific use cases, pie charts are generally poor tools, screwdrivers used to hammer nails. Maybe you’re not the sort of person who has to present data often, or ever–even so, it is useful to know the advantages and shortcomings of so ubiquitous a style as pie charts so that you can knowledgeably assess pie charts that you come across.
Is a 3D pie chart trying to deceive you? Are you dealing with amateurish web or company information that was put together by someone not deeply familiar with the data? These are useful things to get a feel for at a first glance.
It is unlikely that pie chart creators will discover a new use for them.