Have you heard the one about a pie chart?

It’s been almost four years since I attended my first Tableau conference in London and just over 4 years since I discovered Tableau.  I remember sitting in a few of the customer speaker sessions at the conference and I clearly remember one of the speakers joking about using Pie Charts. I smiled thinking I didn’t want to seem like I didn’t get the joke as everyone laughed along!  Then it happened again, another speaker and another pie chart joke!! Fast forward to today and now I’m the one doing presentations for various reasons and I can never resist sneaking in a really bad pie chart to highlight the need for good data visualisation.

So what is the deal with pie charts then? Why are they the ‘in joke’ of the data viz world?! If they are so bad why do they even exist and why is it one of the first types of charts you learn at school?

Most data visualisation books will talk about the use of pie charts and the reason why they get such a bad rep in the data viz world is because the human brain struggles to compare angles. Data visualisation is about making data more accessible and enabling understanding. If pie charts aren’t easy to interpret then they are simply not a good choice of data visualisation.

One of the dangers of pie charts is the use of colour, if you have more than a few slices in your pie then suddenly you have a multitude of colours that might not work together. I have seen so many where there are also similar colours being used and you don’t know which category is which.  I tend to discourage anyone from using pie charts at all especially if they are new to the data visualisation as it is easy to get it wrong. I usually tell people if it’s got more than 3 slices then don’t use pie charts, and if they are still keen to use them to maybe consider using a donut chart instead. The angles in a donut chart are easier to read and you can utilise the space in the centre for extra info, it’s still best to avoid too many slices though.

When I am training new analysts how to use Tableau or running a workshop on data literacy I do tend to talk a lot about pie charts, and I have loads of examples of really really bad pie charts. I personally think it helps to get people to understand what data visualisation is all about.  Rather than just joking about pie charts, I show them why they don’t always work and what other charts work much better.

I haven’t included a pie chart in any dashboard I have built since attending my first Tableau conference. I really struggle with choosing colours that work for my dashboards so I think it’s just better to stick with minimal colour selection and no pie charts!!

So let’s have a quick demo so I can briefly show you what I mean…

Let’s take some little league data using a pie chart to break down the number of runs by 5 different teams:

It is hard to see from the pie chart how much difference there is between the different home team’s runs, the slices all seem fairly similar. You can just about make out which team had the most runs, but you would struggle to see second place.

Now let’s use the same data but presented in a simple bar chart and one colour:

As you can see presenting this same data in a bar chart you can now quickly see that Bears are quite a bit ahead of the other teams, this wasn’t so obvious in the pie chart. If I then sort the bar chart into size order you now easily see which team is in second and third place. That isn’t possible with the pie chart, even if the pie chart had labels on it, it would still take some time to interpret.

I could give you loads of examples to labour my point, and show you alternative charts that work better than pie charts, but I will leave it there! There are lots of great data visualisation books and websites that will explain all this much better than I can. Go check them out!

Thanks for reading my latest blog.

Ella

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