Ah, the pie chart, the Marmite of the data visualisation world. You either love its simplicity or loathe its misleading nature. Just when you think you’ve got it figured out, along comes its sleeker sibling: the donut chart. But which one actually deserves a spot in your Power BI reports?
In my years of working with data, I’ve seen some chart crimes, but few rival the audacity of doubling your total with a pie chart. Yes, you read that right. I’ve encountered charts where the total is added as an extra slice, effectively showing both the sum of the parts and the whole, making the total appear twice. And this isn’t just a rookie mistake. I’ve seen it across industries, from junior analysts to senior executives.
While this particular blunder wasn’t created in Power BI, it highlights a vital lesson: choosing the right chart is crucial. The wrong visualisation can distort insights and confuse your audience, while the right one brings clarity and accuracy.
And this is exactly where pie and donut charts come into play. Both are popular for showing proportions, but they also share a reputation for causing confusion when used incorrectly. So, should you stick with the classic pie or opt for the trendy donut? Let’s explore their aesthetics, best practices, and accessibility considerations.
Pie vs Donut: What’s the Difference?
Both charts aim to show proportions of a whole, but they do it with slightly different flair:
- Pie Chart: The classic. A full circle sliced into segments.
- Donut Chart: Essentially a pie chart with the middle cut out.
Pie charts are best suited for simple comparisons involving up to 4-5 categories, while donut charts work particularly well in dashboards, as they allow the inclusion of a central value (such as a total) to provide additional context.
Aesthetics and UI Design: When Less is More
Let’s be honest, while pie and donut charts can look slick, they often cause more confusion than clarity. Here’s how to avoid turning your visuals into a data disaster:
Keep It Simple
- Limit the number of slices: Pie and donut charts become a visual guessing game when overloaded. If you’ve got more than 4-5 categories, you’re better off with a bar chart.
- Sort by size: Arrange slices in descending order (clockwise). This makes it easier to compare categories at a glance.
- Use direct labelling: Ever squinted at a legend and tried to match colours to slices? Just label the slices directly, it’s faster and clearer.
Pie Charts: The Optical Illusionists
One of the biggest pitfalls of pie charts is how they mess with our perception. Larger slices tend to look even bigger than they are, while smaller ones seem insignificant, making comparisons inaccurate.
For example, if two slices represent 35% and 30%, the larger one often appears disproportionately huge, even though the difference is only 5%. Donut charts slightly reduce this issue, as the hole in the middle helps normalise the perception of size.
Accessibility Considerations: Don’t Leave Anyone Behind
Pie and donut charts can be striking visual tools, but their heavy reliance on colour and shape can pose accessibility challenges. Before choosing these chart types, consider the following factors to ensure they effectively convey your message while remaining inclusive.
Colour Contrast Matters
In data visualisations, colour contrast is key. Aim for at least WCAG AA compliance:
- Text and data labels need at least a 4.5:1 contrast ratio against their background.
- Non-text elements (like chart slices) should meet at least 3:1 contrast.
- Avoid using red-amber-green (RAG) colours, these are a nightmare for colourblind users. Instead, use accessible palettes like blue-orange or purple-yellow.
If you’re new to calculating colour contrast ratios, check out my YouTube tutorial: Power BI: Colour Contrast Delight.
Use Clear Labelling
Labels provide direct context by showing the exact values or categories each slice represents, removing the need to rely solely on colours or shapes to interpret the chart. Con sider the following:
- Data labels directly on the chart improve readability, no need to keep referencing the legend.
- Include percentage values next to the slice names for instant clarity.
Add Alt Text and Annotations
In Power BI, alt text enhances accessibility by enabling screen reader users to understand your visuals. For instance, you can use alt text like:
“Donut chart showing revenue distribution by region. Europe accounts for 45% of the total, followed by North America at 30%. Smaller segments represent Asia-Pacific and Latin America.”
To make alt text more dynamic, consider using DAX for conditional formatting. However, keep in mind Power BI’s character limit of 250 for alt text, and ensure any changes to the visual are reflected in the DAX expression.
A helpful tip is to include the option to ‘Show as table’, which is screen-reader friendly. For example, the alt text could be revised as:
“Donut chart showing revenue distribution by region: Europe, North America, Asia-Pacific, and Latin America. For a screen-reader friendly format, use the shortcut Shift + Alt + F11 to view Data as Table.”
If you are unfamiliar with this option in Power BI, here is how you can find the ‘Show as a table’ option.
Additionally, use annotations or header ‘Help tooltip’ to provide extra context without overwhelming the visual. However, bear in mind that ‘Help tooltip’ is not compatible with screen readers, so consider alternative ways to ensure inclusivity.
Common Mistakes to Avoid
Even when pie and donut charts are used appropriately, some common pitfalls can undermine their effectiveness:
- Too Many Slices: Including more than 4-5 categories can make your chart messy and hard to interpret.
- Tiny, Unreadable Slices: If your chart contains several small categories, consider grouping them into an ‘Other’ category to improve clarity and readability.
- 3D effects: While I haven’t encountered 3D pie charts in Power BI, some visuals may allow for them. However, they should always be avoided, as 3D effects distort proportions and compromise the accuracy of your data.
By addressing these challenges, you can ensure your charts remain clear, professional, and accessible.
Wrapping up
In the world of data visualisation, both pie and donut charts have their strengths and pitfalls. The choice between the two ultimately depends on your data, your audience, and your accessibility goals. While pie charts offer simplicity, donut charts add a modern touch with space for extra context. But remember, a poorly designed chart, no matter the type, can obscure insights rather than reveal them. So, which will you choose to make your data shine: the classic pie or the trendy donut?
Thank you for joining me on this journey. Until next time, let’s keep crafting accessible insights that make a difference!
Generally, I avoid them almost 99% of the time: Pie Charts or Donut Charts, or Surface Charts as they provide minimal value in terms of Story Telling. They do visualize the data but don’t tell much of a story. Great post, anyway, with a lot of thoughts!