Why Birthday Cakes Mean That The Pie Chart Will Never Die
By: Tessica Dall
Tags: Pie Charts, Charts, Data, Data Visualization

Why Birthday Cakes Mean That The Pie Chart Will Never Die

Data visualisation gurus passionately advocate against pie charts, but the masses will never be convinced — nor should they be.

Image by Ruth Black on iStockphoto

It is almost a rite of passage, perhaps even seen as a professional responsibility, among those working in data visualisation to voice their distaste for pie charts and to be seen to broadcast stern warnings for them to avoided at all costs. I am not convinced. Indeed, my presentations and dashboards include them and I teach others to construct them in my training courses. Am I a rebel, a pragmatist, or is there something fundamentally wrong with the message that pie charts are ‘evil’?

Why all the hate?

Edward Tufte, author of the data viz bible ‘The Visual Display of Quantitative Information’ and someone who was once described by the New York Times as “the daVinci of data” once said:

“the only thing worse than a pie chart is several of them” — Edward Tufte

He is not alone in his views. Countless books, articles, and blog posts by data visualisation leaders promote the same message paired with equally passionate titles. Check out “Death to pie charts” by Cole Nussbaumer Knaflic and “Save the pies for dessert” by Stephen Few.

Their argument is that the human brain is not wired to measure angles (the basis on which pie charts are constructed).

Image by LibreTexts Mathematics Library

Our brains are designed to quickly process information and assign meaning to it, but the types of attributes it uses to do so are things such as similarity, proximity, and enclosure. They’re referred to as pre-attentive attributes or the gestalt laws of perception and Elijah Meeks wrote a good series of articles on how these apply to data visualization — check it out here. There are 7 attributes and angles aren’t one of them.

It is this that prompted Walter Hickey, data journalist and Senior Editor, Data at Insider, to open one his blog posts with:

“The pie chart is easily the worst way to convey information ever developed in the history of data visualization.” — Walter Hickey

Measuring angles is hard. Measuring and comparing multiple angles is even harder. I won’t argue with that. It’s why we learnt to use a protractor at school and why none of us would contemplate constructing a pie chart by hand.

But…

Your brain didn’t know how to assign meaning to letters, words and sentences when you were born — you were taught (and I assume you have reached a pretty decent skill level in this regard else you wouldn’t be reading this article). The same is true of pie charts, albeit in a different context.

Why I think pie charts are great

From the moment we are born our families press on us the importance of celebrating the anniversary of our birth, and those of the people close to us. Central to this celebration is always the birthday cake. We quickly learn that the size of the slice is an important metric. Quantity wins over quality every time as a kid, so quickly identifying and nabbing the biggest slice provides a competitive advantage. Also important, if you have siblings, is ensure that whichever slice we get is at a minimum bigger than that of our siblings. Or if you’re born into a family of control freaks, as I was (and where I was perhaps the biggest culprit), you’ll get the ruler or protractor out to make sure everyone gets the exact same sized slice.

This puts us in good stead for a future business context, when faced with data presented in a pie chart. As long as the slice sizes are noticeably different and there are not too many of them (remember that our brains can only retain about 5 bits of information at once), then we can do a good job at interpreting the data.

Stephen Few once said:

“Data visualization is the graphical display of abstract information for two purposes: sense-making and communication.” — Stephen Few

I believe this really is crux of the argument. Data is presented in order to convey information to an audience. True, as a method to present data, the pie chart is one of the more complex and difficult for our brains to process — but just like human language — we have practiced it since birth so we understand this type of visualisation innately. It is this familiarity that outweighs the gestalt laws and lifts pie charts to one of the most valuable chart types (alongside bar charts) for communicating data to business audiences.

(bring on the internet haters and trolls…)

I am no rebel, I accept the intrinsic flaws of pie charts, but I am also a scientist who trusts observations over theory. In my 20 years experience presenting data and teaching data visualisation — audiences are more baffled by cognitively simpler alternatives to pie chart types such as tree maps and waffle charts (as much as I love them). It is no accident that bar charts and pie charts are the default and most prolific of all chart types.

When precision is key, chose a bar chart — it will always do a better job for comparing values, but when the data and point you’re trying to make allows for it, feel free to use pie charts.

That is not to say that I am advocating for free reign with pie charts, or their cousin, the donut chart. The easiest way to get me to lose interest in your data is to present a 3D, exploded pie chart, but that’s a whole other story. It is critical to be mindful of good information design — all elements of the chart must support effective communication of the information of the audience.

Image by various via Google

Image by various via Google

Designing your chart, regardless of its type, for maximum clarity and communication is key. Pie charts are not ‘evil’, do not need to be avoided, and can actually be a great asset in your data visualisation toolbox as they are easily consumed by your audience (pun intended).