The Pie Chart Dilemma

A pie chart is used extensively in presentations and reports. I use pie charts very often in my monthly performance dashboards. Or to be fair I used to.

The Naughty Pie Chart

In some of my recent videos I have used pie and doughnut charts without putting much thought.

If you watch my eCommerce Dashboard video you would agree it is hard to understand the pie chart I used for eCommerce conversion rate by country. The main reason being that the data contains many categories (countries) (slices) and a pie chart is totally unsuitable for the visualization of this type of data.

In fact it’s ugly!

So lately I have been very cautious about how I use pie charts. Or even doughnut charts!

Against the Pie Chart

Regardless of their popularity I have heard many data visualization experts to speak against pie charts and recommend using bar charts instead. The argument is that pie charts are hard to read.

When pie chart slices are close in values it is hard to determine which one is bigger. If the values are not close in size you can only determine which one is bigger but not by how much.

With bar or column charts however the audience will compare the end points. And because the values (size) of the bars or columns start at a common baseline it is easier to understand their relative values.

Also as discussed above pie charts are not suitable when you are comparing the values of more than a couple of categories.

In the visual below the pie chart is used to visualize traffic by marketing channel to a certain website. In this example we have seven categories included in the chart, each represented by a slice in a different color.

pie chart

Looking at this chart I have to agree it is hard to analyze each channel’s performance as compared to the other channels.

Below is the same data visualized with a bar chart. And it does look much better! It tells a good story that is very easy to understand.

pie chart vs bar chart

In Defense of Pie Charts

But pie charts do serve their purpose!

In the example above we are looking at seven different categories. But if we are visualizing only two parts of a whole a pie chart doesn’t look this bad. In fact I think I prefer it over a chart with only two bars.

Here is an example of a data visual for new vs returning visitors to a website.

pie chart two values

vs:

 

pie chart vs bar chart two values 1

Another big reason I have stuck with pie charts for a long time is that they are designed to compare parts of a whole. Or in other words they are the visual that will show percentage data labels.

All of the data visualization tools I have worked with only pie charts will calculate the percentage of each part to a whole. Including Excel.

Unless your numbers are already calculated and your tables include percentage values you will not be able to assign a percentage data label to a bar chart. (correct me if I am wrong here!).

Final Thoughts

Your lust for pretty data might lead you away from pie charts. And that’s understandable. But they can often be more effective if you are comparing two categories (two slices) within a total.

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