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