Monday, 24 June 2013

Having your pie and eating it

This week I was asked about pie charts, not perhaps something you are asked every day, but a common enough question for an analyst and one you need to have a ready opinion on if you’re going to count yourself as a data professional. A tweet had started the debate and everyone likes to share their views so here are my musings.

Over the years, I’ve been a supporter of the “no pie charts” campaign in the past but more recently I’m mellowing. I’ve seen 3D pie charts (still a no-no for me), pie charts with more pieces of pie than you could possibly count, pie charts that don’t start at zero (12 o’clock) and pie charts with no legend or title. All of these still get my goat somewhat. However, I still use pie charts and here’s why. Everyone knows what they are, the fact that they are now on every food packaging and label you find means people are used to seeing them and used to reading them. Moreover, they do show at a glance the “biggest” slice of the pie. Even a small child can say which bit of pie they’d rather eat, so it makes it easy to see the largest proportion. 

More important, is the fact that you can also combine slices of the pie visually and see what proportion you get, varying the categories to include.In the “Bad example” (pie chart) you can easily see that organic searching, email marketing and social media takes up more than ¾ of the pie. It’s a rough and ready reckoner for people who just want to combine different parts of the data without having to read axis with small printed labels. Compare this with the “recommended graph” (bar chart). It’s not so easy to combine the 3 categories. Here you have to add up in your head 48%, 19% and 17% - anyone? (No cheating with a calculator!).

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 My husband (also an analyst) came up with a different anti-pie chart argument that I hadn’t heard before. Edward Tufte’s Ink to data ratio. This states that “A large share of ink on a graphic should present data-information, the ink changing as the data change”. This isn’t a campaign to save printer ink. The Non-Data-Ink is the ink that does not transport the information but it is used for scales, labels and edges etc. Pie charts don’t fare well here either as there’s lots of non data-ink around a pie chart.

My last hope to save the humble pie chart, is to say they’re simple to create. Excel does them for you with the push of a button. This may not initially be seen as a positive, but as a supporter of life-long learning I know that there are people out there who run away, screaming at the thought of numbers, graphs and charts. For the data-phobic, creating a pie chart may be a big step and the first they’ve taken to turning raw data into some visual analysis. It may not always be pretty, but it is a start. So there you have it, I’m not going to stop using pie charts, but I’m not going to start recommending them either. Sitting on the fence perhaps, but eating a large slice of pie.

The views on this blog are the views of the author, Emily Harper, and not necessarily the views of Citizens Online.