Bottom Line: This post explains best practices for using pie charts in your reports and dashboards based on common data visualization principles. You will also learn techniques for picking the best slice of pizza/pie. 🙂
Skill level: Beginner
Pie or Not?
When it comes to the art & science of data visualization (aka principles for creating charts & dashboards), pie charts are the most controversial topic.
Some experts will tell you to NEVER use pie charts. Others say use them sparingly, and usually have a strict set of rules to follow.
So, when do we use pie charts?
The Balance of Supply & Demand
The decision on when to use pie charts usually comes down to a few factors.
If you are familiar with the principles of data visualization, then you might have had a conversation like this before…
Boss: “This dashboard you created looks great, but we’d like to have a few pie charts on it.”
You: “Thanks Boss! However, we really shouldn’t use pie charts. They can be difficult for the reader to interpret. Our eyes have trouble comparing the size of the slices…”
You go on for about 7 minutes, quoting experts like Stephen Few and Edward Tufte, trying to explain how pie (charts) are bad for your (dashboard’s) health. You might even show some examples from books or websites.
During this rant your boss nods his/her head like they’re in agreement. You feel like you are really making some progress.
He/she then replies with, “Yeah, but we’re going to need you to add some pie charts to the dashboard. Can you get it done by Thursday?”
If you have experienced something like this, then I applaud you for trying. I have tried and failed before too.
But don’t let this get you down!
The reality is that the use of pie charts comes down to two forces:
- Supply – Our knowledge of pie charts and when to use them.
- Demand – The requests from our audience (usually our boss and/or their boss) and the desire to please them. We still want to keep our job, right?
In the situation of pie, demand usually wins. So let's look at some best practices for creating pie charts.
The Rules for Pie Consumption
This is where things get ugly. There is a lot of controversy over when to use pie charts. It’s a highly subjective matter that some people get angry over.
So, I’m going to explain a few common rules to help keep you safe. I will also share some resources if you really want to dive into this topic.
Just remember that at the end of the day, the decision can come down to the conversation we had with our boss above. They might even have a valid point…
What are Pie Charts?
Pie charts are used to display the percentage of total.
All the slices (segments) must add up to 100%. Any individual slice will display its portion of the whole.
The following chart shows the revenue for each region as a percent of total revenue. The sum of all regions should always equal 100%.
The circular shape of the pie quickly conveys this message to the reader. We are dividing the pie into slices and can’t have more/less than 100%.
Note: The data labels might not add up to 100% exactly if you are rounding to whole numbers.
When using a bar/column chart for percentage of total, it is sometimes more challenging to quickly determine that all the bars add up to 100%.
The pie chart gives us a quick win here. This use case seems simple enough, so let’s see where things can go wrong.
Comparing The Size of Slices
The biggest issue with pie charts is that it is very difficult for us to interpret the size of the slices.
Based on the triangular shape of the slice, it’s tough for our brain to compare the size of one slice to another. Especially when the slicers are not next to each other.
This comparison is easier with a bar or column chart.
Of course, the pie chart would be easier to read if it had data labels for the percentage values. But that defeats the purpose of using the chart to make quick visual comparisons.
When Pie Gets Messy – Too Many Slices
The fact that the slices are confined and laid out in the circle makes this even more challenging. The more slices you have on the chart, the harder it can be to read.
There is a lot of debate on the maximum number of slices. Most experts say 4-6 is good, with 10-12 being the absolute max.
I think a lot depends on the data and the story you are trying to tell.
If you want to show that one slice is bigger than the other 10 combined, then you might be able to get away with more. That would mean that the values of the smaller slices are less important, and the overall message is that the large slice is dominating the story.
Most viewers are still going to want to see the details of the other slices, even if they aren’t relevant. So, I call this one a questionable use of a pie chart.
The overall recommendation here is to keep the number of slices to a minimum. This makes it easier for your readers to compare (pick) slices.
Picking Your Slice – The Greatest Argument in Food
Have you ever had an argument with a sibling or friend on who gets to pick the first slice of pizza? This is an age-old battle, and sometimes a lot of the fun of sharing a pie.
There is a lot of pressure on the person that gets to go first. Will they be able to figure out which slice is the biggest?
Depending on how much beer is being consumed, a heated discussion will typically arise from this. Of course, toppings also come into play, but let’s just focus on size for now.
A lot of times the final judgement will come down to laying one slice on top of the other (stacking them). Here's an image from our previous example that shows this comparison.
This proves just how difficult it is to determine the size of a slice when comparing it to other slices.
The Secret to Picking Your Slice
I will give you a secret insiders tip on how to pick your slice… 😊
The trick is to be directly over the center of the pie.
This gives your eyes the best shot at comparing slices. It’s also the reason 3D pie charts should be avoided.
Don’t Use 3D Pie Charts, Ever!
Ok, that is probably the boldest statement I’ll make in this post. But it’s only because I want you to have the best chance at picking the biggest slice. And also presenting your data in an honest way.
3D charts, especially 3D pie charts, are very deceiving.
When we create a 3D chart, we are now using 3-point perspective and a diminishing horizon. This means the objects in the foreground will be BIGGER than the objects in the background, even if they aren’t actually bigger.
Here is an example.
Which slice do you think represents the larger data point?
I’ll try to give you a hint. The surface area of slice #1 is much larger.
Unfortunately, #2 is the winner here. But it’s stuck in the back and looks much smaller due to the perspective and diminishing horizon.
The slice in the foreground is smaller in terms of the size of the data. However, the shape that represents the data measures larger than the “bigger” slice in the back.
The more you angle the chart, the worse this distortion gets.
And exploding 3D pies only make this worse, as the foreground pieces get farther from the background ones.
So now you know. When choosing your slice, get directly over the pie!
You are also doing your audience a disservice by serving them 3D pie charts. It is much more difficult for them to read these.
We must use extra labels to describe each point. This can slow down the reader and defeat the purpose of the chart, which is to tell an honest story about the data.
And before you get angry with software vendors like Microsoft for including these options in the applications (Excel), just realize they are dealing with the same demands you are. Only multiplied by hundreds of millions of users. Now I agree the 3D charts shouldn’t have made it in there in the first place, but it’s going to be much harder to remove them now that demand has been created.
Remove the Legend
Another issue with pie charts is the use of legends. In the following example the legend is below the chart.
This is really going to slow the reader down. Our eye must travel back and forth between each slice and the legend.
It’s better to put the category name in the label. This is easy to do in Excel.
The legend is typically added when we use a lot of slices and don’t have room on the chart to display them. If this is the case, then it’s a good indication you should NOT be using a pie chart.
A bar chart might be a good alternative.
Don’t Present Two or More Pies
Let’s go back to the decision of picking the biggest slice of pizza. Now imagine if you have two or three pizzas sitting on the table side-by-side. How difficult is it going to be to pick the biggest slice now?
The same is true with pie charts on a dashboard. It’s going to be very difficult for the reader to quickly see the changes between all the slices.
We are usually making these comparisons over time. What is the portion of sales for each region this year versus last year?
Here is an alternative using a line chart that shows the percent of total revenue for each region over 3 years.
For more alternatives to multiple pie charts checkout this great article by Stephen Few titled Save the Pies for Dessert (link to PDF). He also explains similar reasons on when (not) to use pie charts.
Why our eyes like circles?
If pie charts are so terrible for us, then why do we like them so much?
Well, it turns out our eyes are drawn to circular shapes. Here is an article by Manuel Lima on Why do we find circles so beautiful that explains more. Manuel has also written a book on the topic.
He boils it down to three main reasons.
- Circles give us a feeling of safety. It’s harder for someone to hide in a dark corner of a circular room.
- We are drawn to the round shape of the human faces. The more round and symmetrical features, the better. Think cute baby pics.
- The eye itself is round. And for that matter, so is the planet we live on and the sun that provides us energy.
I also feel like circular shapes drive curiosity and exploration. For example, balls roll, bounce, and can be unpredictable in movement. Try golfing, bowling, or any other sport!
This leaves us with a since of wonder, and that these shapes can almost defy gravity sometimes.
Our charts & dashboards are typically made up of a lot of rectangular shapes that are laid out on a grid (Excel sheet). So, it makes sense that we might want to add some circles to our dashboards.
I think we can even use pie charts strategically to draw the readers attention to the dashboard (a topic for another post…). I’m sure this was the bosses argument at the beginning, they just didn’t know how to articulate it… 😉
Conclusion – Use Pie Like It’s Pie
So hopefully this post has helped you see why the boss is demanding pie charts. They aren’t necessarily evil. However, when we are telling the story of data with charts, we want it to be an honest one that is easy to consume.
The main rules and best practices for pie charts are:
- Keep the number of slices to a minimum.
- Use a bar/column chart when the data points are close in value. It is hard for our brain to judge size differences of triangular shapes.
- Don't use 3D pie charts, ever.
- Don't use multiple pie charts to make comparisons.
- Remove the legend and put the data labels on or outside the chart slices.
- Do your best to educate your boss & audience, but don't get too upset when demand wins.
- Use pie charts sparingly.
Telling the story with pie charts can be challenging. So, I recommend that you use them sparingly. About as often as you “should” eat pie in real life. We all know we shouldn’t eat pie every day, and the same is true for creating healthy dashboards.
Ok, I know I used a lot of food puns in this article, but I think they make good metaphors for using pie charts.
And you’re now equipped with some pretty good slice-picking skills for the holidays! 🍕🍰
Here are some additional resources on pie charts and data visualization. This will help you learn the right chart types to use for your data, and design principles that make them easier to read and understand.
- Stephen Few (Perceptual Edge) – Article on Pie Charts | Website | Books on Amazon
- Edward Tufte – Website | Books on Amazon
- David McCandless (Information is Beautiful) – Website | Books on Amazon
- Cole Nussbaumer Knaflic (Storytelling with Data) – Website | Books on Amazon
- Article and Video on How to Create Progress Donut Charts with Conditional Formatting
- Excel Dashboard Course by MyOnlineTrainingHub
- Power BI Dashboards & Data Course by Excel Campus (that's me)
There are a TON of resources, books, and courses on data visualization. This is by no means an exhaustive list, but should help get you started.
Please leave a comment below with any questions or suggestions. Thanks!