Donut chart data visualizations are useless!

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visualization-32988_1280Let me just start out by saying that donut chart data visualizations are useless, in my humble opinion, and in this post I am going to try explain why I hold this view.

Before doing that let I want to state that the use of data visualizations can be very powerful when used correctly. Data visualization can help you “see” things in your data which conventional tables could not so easily expose.  There is absolutely no doubt that a collection of good data visualizations deliver information more effectively than a list of tables. There is also no refuting the fact that a lot of data can be displayed on a great data visualization allowing an overview which is just not possible via tables. At the same time there are downsides especially if  poor data visualizations require more time to understand than either a table or a different form of data visualization.

So lets start out by saying the aim of a good data visualization is to quickly show you the data you need to see without having to read lots of text on the screen. This is why the disturbing trend of donut charts taking hold is not a good thing. Out have gone pie charts, which incidentally are also pretty much useless, and in have come so called donut charts. A donut chart has all the problems of pie charts but now we can also write a number in the middle.

Here is a basic donut chart. I have on purpose shown this without any data value labels.

DonutBrowserNow I challenge you to quickly answer these questions:

  1. Which value is the smallest?
  2. Which value is the largest?
  3. Which one is the 4th largest segment?

Answering these questions takes too much time because of the nature of a donut chart data visualization. This is especially true of the 3rd question in the list.  The simple fact is that showing that same data in a table or even as a bar chart would have made answering these questions very simple. The other thing that would have improved things would have been to order the donut chart so the largest segment came first and the smallest last. Still it is not quick to answer those questions even with those things done.

Now, using different data, try answer the same three questions by looking at this bar chart data visualization.

barchart

Even without ordering you can much more quickly answer the questions. This is simply because a bar chart is a better vehicle for presenting this sort of data.

Ok, but how does this change if we add labels and percentages to the donut chart?

Dnut_labels

As you can see this donut chart improves on the first one as it has percentages listed for each slice. If I were to ask the same three questions I asked earlier, thanks to the labels, you can probably more quickly answer them. The issue remains though that it is the TEXT labels and not the data visualization that you are reading to answer the questions. This means that the data visualization itself is not really adding a lot apart from making your text look more pretty.

In conclusion

I am a big fan of data visualization. I also understand that when a customer asks for a specific type of data visualization it often needs to be provided otherwise they will go elsewhere. Many data visualization tools have evolved to support pie charts and most are now adding donut charts driven by the desires of their customer base.

Having said that people designing reports and dashboards need to ask themselves if the data visualizations (and this is not just limited to donut charts) they are using adds anything except colour to what they are delivering.

If 50 people need to use an extra 30 seconds to answer the three basic questions every day then you are wasting 25 minutes per day (which assuming a 300 day working year is 125 man hours per year or around 16 man days) .  If that number goes up to 1000 people then that becomes just over 8 hours per day that are being wasted. That calculation is on a single data visualization.

Pay attention to your data visualizations. They could actually be costing the company money even if they look pretty! I hope now you can see why I say donut chart data visualizations are useless even if they make things look more pretty sometimes!

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