How NOT Knowing Pie Charts Makes You a #DataViz Rookie
If I live to be 115 years old, I’ll never understand why consultants use pie charts. Your chance of doing them wrong is greater than doing them right. At the SAS Global Forum last week, I was delighted to hear several others telling me how much they hated pie charts and we agreed, they need to be stomped out – we are forming a #DataVizArmy to do just that. Our first mission is to rid the world of poorly done pie charts.
Guidelines for Using Pie Charts
Here’s the guidelines for how to properly use a pie chart to display your data. I use the word guidelines because there are times (few and far between) that it makes sense not to do it this way.
Remember: Pie charts are used to show the parts to the whole.
- Parts to a whole equals 100% – always
- Limit to 4 or 5 categories
- Doesn’t need a legend when done properly
- Better when one category is significant percentage-wise
- Not suitable for comparisons
When It Looks Good
Here’s examples of good pie charts. Notice that not having the percentage doesn’t matter that much – you know that Tracy is spending too much time trying to find the right content on Netflix and not enough time taking care of business. (Names changed to protect the lazy.)
I like that the author went to the extra trouble of noting why the values may not equal 100%. Maybe having the percentage makes it seem more scientific?
Rookies Love Chart Junk
When you don’t understand the guidelines above, you are at greater risk for creating chart junk. Let’s look at some examples of when these guidelines were not followed to further convince you.
Is your data suitable for a pie chart?
Remember – the data is supposed to be the parts to a whole. What is the author’s point is with this pie chart? There’s not any numbers here – just dates and drug names. This chart only informs me that anti-coagulants were introduced in 1982 and more have entered the market since that date. It doesn’t express perhaps the different categories, the popularity, or anything really. Use a line chart when you want to show a timeline or a simple line that shows the dates.
Legends might make the pie chart superfluous
If you are thinking of adding a graphical element to the page, this is where you fall into the trap. Really question yourself if you start reaching for a pie chart – that’s a rookie move. For instance, does this chart really add anything besides some color? I bet you just read the numbers and didn’t even notice the chart.
Famous example of Breaking the 100%
Again ask yourself does this data make sense as a pie chart? Just about anytime you read about pie charts – you are likely to see this example. It’s hard to believe someone was so confused. It’s back to that need to show data visually but not understanding what you are doing. Are you glad they listed the percentage – otherwise the fault when have gone unnoticed.
Limit the categories to focus the reader’s attention
When you have too many categories it makes it harder for the reader to understand your point. The reader may ask themselves “Is this a ranking?” or “Do these other categories really matter – why am I being shown this?” Again notice how going back and forth between the colors and legend is a drag. This would make more sense as horizontal bar chart.
It’s Hard to Compare Pies
Avoid using pie charts to compare to things – it’s harder on your reader. The reader had to keep a lot of information in their visual memory – the order of the categories and the values. So the comparison is really difficult and taxing.
Review the difference when I rebuilt the chart in SAS Visual Analytics. I used a butterfly chart, which is really just two bar charts back to back. It’s a lot easier to compare the numbers and more clear how different the spending is. Maybe you didn’t notice before that rich don’t spend a lot of their money on tobacco or perhaps it’s not a large enough percentage to register. They do spend more on personal insurance and pensions!
More Rookie Alerts
What do you think the message is … “I don’t know what I’m doing?” or “Watch mom – I can do colors.”
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Tags: Data Visualization Tips