Introduction
When it comes to visualizing data, pie charts and donut charts are two of the most common go-to options. They’re simple, visually appealing, and help you present data in a way that’s easy to digest—pun intended! But when should you use a pie chart? Or a donut chart? In this guide, we’ll break it all down for you, so you can confidently pick the right chart for your next project, and show you the best practices.
What is a pie chart?
A pie chart is one of the most widely recognized types of data visualization. It's a circular chart divided into sections, or "slices," where each slice represents a proportion of the whole. The size of each slice corresponds to the percentage or value it represents, making it easy to see how different categories contribute to a total.
Key Features and Components
- Slices: Each slice represents a category or value, with its size proportional to its percentage of the whole. The more significant the value, the bigger the slice.
- Labels: These are used to describe what each slice represents. In some cases, they may include a percentage or the raw data value for clarity.
- Percentages: Often, the percentage that each slice represents out of 100% is displayed directly on the chart, giving viewers an at-a-glance understanding of the data distribution.
Advantages
- Easy to Understand: Pie charts are easy to understand, particularly for non-technical audiences.
- Proportional Representation: They effectively show relative percentages of a total, allowing viewers to quickly assess how parts contribute to the whole.
- Simple Design: A well-designed pie chart with a limited number of categories is easy to interpret at a glance.
Disadvantages
- Limited to Few Categories: When you have too many categories, pie charts can be difficult to interpret, because the segments are too small.
- Difficult to Compare Segments: Pie charts are best used for showing proportions. However, for comparisons between segments, it’s not always easy to see the differences, because humans don’t judge angles very well.
- Precision: Pie charts don’t allow for precise values to be communicated, especially when comparing percentages. Labels and percentages are often necessary but can clutter the chart.
Donut Chart
A donut chart is essentially a variation of the pie chart but with one key visual difference: it has a hole in the centre. Like pie charts, donut charts divide a circle into slices that represent different categories, but the hollow centre offers a bit more flexibility in terms of design and information display.
Key Differences from Pie Charts
- The Hollow Centre: The most obvious difference between a pie chart and a donut chart is the empty space in the middle. This space can serve both aesthetic and functional purposes. For example, you can use it to add a label or an additional data point, like the total sum of the categories.
- Emphasis on Categories: Because of the hollow centre, the focus in a donut chart is often more on the proportions between categories rather than on the chart as a whole. The empty space helps reduce visual clutter, making it easier to compare slices.
Another added advantage of Donut Charts is that they can layer around each other, or around a pie chart.
This type of chart is called a nested donut chart, or a nested pie chart, if the inner chart is a pie chart.
Best Practices
1. When to use a Pie Chart (Or a Donut Chart)
A pie chart should be used only when you have a whole amount, that is divided among different parts.
The above pie chart is a good example of what NOT to do. The numbers don’t add up to 100%, so it is misleading at best. In addition, it is also difficult to differentiate between the segments.
The eye is well suited for distinguishing the differences in length in a bar chart but struggles to compare the area of a wedge.
2. Avoid using a pie chart when values are similar
As you can see in the above charts, a bar chart illustrates the differences between segments much more easily than a pie chart.
3. Don’t have too many values on a pie chart
Too many values make a pie chart look cluttered. It’s good practice to bundle the smaller value into an “others” category”. The exceptions to this rule are when you want to highlight every category mentioned, or when you want to emphasize the variety of categories.
4. Pie charts should add up to a whole
Circles are a universal metaphor for completeness, and should contain all the data. We see this rule being broken mosty often when people are allowed to choose multiple options from a list, such as pizza toppings…
Or when a business grows more than 100% (or less).
The only exception to this rule is in the rare case you need to show numbers rather than percentage:
5. Pie charts should start at 12 o’clock and go clockwise
Humans are used to reading clock faces, so intuitively can read these charts better.
5. Ordering
By default, order your pie charts from largest to smallest, going in the clockwise direction of course.
There are exceptions to this rule, however. The most common exception is when your data is ordered or sequential, as shown in the chart below:
6. Avoid exploding pie charts
Exploding pie charts are typically done to highlight one category. However, it’s almost never a good idea for an exploding pie chart. Not only is it harder to read and interpret, it doesn’t look very good either. The only time you’d want to break this rule is to emphasize when a part has been broken from the whole:
7. Colour Usage
Be conservative with the number of colours you use in a pie chart. The usage of colours is strategic. For example:
- Use different shades of the same colour to convey kinship. Use 2 sets of analogous colours to suggest 2 different families.
- Use colours associated with the categories when applicable
- Use a single, stand-out colour to highlight a standout category, using muted colours or greyscale for the others:
- When all your pie chart slices are important and represent different categories, you’ve got to use colours that stand out from each other. But you don’t have to go for clashing combos like red and green or purple and yellow—those can be a bit much. Instead, try picking colours that are spaced out around the colour wheel, like blue, red, and orange. This way, your chart stays vibrant but easy on the eyes.
Your brand’s colour palette probably has enough options to make this work, and if it doesn’t, you can dip into the secondary palette for extra variety. Just remember, never reuse the same colour on a pie chart—it’s a surefire way to confuse your audience!
8. Avoid 3D Pies
3D is never a good idea when it comes to visualisations, and pie charts are no different. The human eye sees in 2 dimensions, not 3.
Alternatives to Pie Charts
1. Tree Maps
Similar to pie charts, tree maps are also used to show how different parts make a whole. Tree maps are considered a better alternative when:
- There are more than 6-7 categories
- When the differences between categories are small
- When you are displaying multiple levels of data
2. Waffle Chart
Waffle charts are a lot like tree maps. A waffle chart comprises 100 icons, typically squares laid out in a 10 x 10 grid. Each icon represents 1% of the data. The drawback of this chart is the lack of space for labelling.
3. Stacked Bar Chart
Stacked bar charts are a viable alternative to pie charts when:
- You want to compare totals as well as composition
- You want to show change in composition over time
Conclusion
Pie charts are far from perfect. Their use case is fairly limited, and they can very easily be misused. However, the pie chart excels as a simple, easily interpretable chart when you have a single data set with a few categories.
A donut chart adds a level of perceived sophistication to a pie chart, and offers enhanced flexibility. For a world where data drives decisions, pie charts and donut charts remain indispensable tools. Use them wisely, and let your data visuals speak volumes!
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