How Succinctly Can I Explain Why Pie Charts Are Evil?
By Tim Wilson on in Data Visualization with 11 Comments
I’m right at three months into my new gig, and, around the office, probably the most commonly known fact is, “He hates pie charts.” It’s not that I’ve exactly been standing at the elevator handing out leaflets explaining why pie charts are evil, but I have, apparently, chosen a couple of particularly public venues to make a mild statement or two. And, the quasi-preplanned visceral groan when some co-workers put up a pie chart might’ve contributed just a teensy bit.
I’ve been put on the spot since then a couple of times to do one of two things:
- Explain why pie charts are evil, or
- Agree that one or another particular usage of a pie chart is appropriate
After catching up on some blog reading yesterday morning and seeing an excellent example of pie chart alternatives from Jon Peltier, and then watching seven presentations yesterday, six of which used the same basic presentation template, and five of which stuck with a pie chart for the sole non-text slide in the presentation, how could I not write another post?! Let’s see how succinct I can make it (don’t hold your breath that you could read the whole thing before exhaling!).
Yes, There is ONE Thing That a Pie Chart Does Well
This kills me, because there’s one way, in a a very narrow set of circumstances, that pie charts do marginally better than alternatives. All THREE of the following criteria have to be met for this to be the case:
- Exactly 2 or 3 categories that make up the “whole”
- A fairly significant difference in % makeup for each of the categories
- Plenty of space available to present the information
99 times out of 100 when pie charts get used, all of these criteria are not met. But, there, I’ve admitted that there is a situation where pie charts are appropriate.
Of course, mullets are an appropriate hairstyle if you are prone to both warm ears and spontaneous hair donations…but that doesn’t mean I’m going to sport one!
Of Course, We Must Start with a Before/After Example
With only the category names changed, below is one of the pie charts I saw yesterday:
In my experience, a simple horizontal bar chart is a better option (among a variety of better options):
Why is this a better option? Oh, let me count the ways…
1. Rainbows Are Good in Princess Tales — Not in Data Visualization
When it comes to data visualization, a chart that doesn’t rely on multiple colors always trumps a chart that does. Four reasons:
- If you use subtle/muted colors, you can’t get past 4 or 5 categories before you are asking the person reading the chart to work hard to distinguish between subtle shading differences
- If you use bright/high-contrast colors, you’re asking your user to put on sunglasses to keep from wincing at the visual overkill
- Roughly 10% of men suffer from some form of color-blindness — it’s darn tricky to nail a palette with more than a small handful of colors that works across the various types of the condition (of course, if you’ve got a secret agenda to have women take over the world, this is one way to contribute, as color blindness is exceedingly rare in women)
- Maybe you’re presenting your chart in glorious, projected color…but are you sure no one is going to try to print it in black-and-white?
These are all issues with any pie chart that has more than 3 categories. None of these are an issue with a horizontal bar chart.
2. Labels, Labels, Labels
If you’ve every constructed a pie chart in Excel, you’ve run into the challenge of trying to get all of the wedges labeled right there on the chart. Excel continues to make odd choices as to where to wrap text in pie charts, and the circular nature of the whole layout means some wedges have plenty of horizontal labeling room, while others have almost none. You’ve tried some (or all) of the following:
- Using leader lines for some of the wedges so you can label the most troubling wedges somewhere more spacious
- Abbreviating the category names
- Strategically rotating the chart so that the labeling all happens to work (it never does)
- Rearranging the underlying data so that the pie wedges occur in a different order (which also never works)
After fiddling with the above, you finally break down and yank the labels from the chart and just use a legend. This is bad, bad, BAD! Scroll back up to the pie chart example above and pretend you’re actually trying to interpret the data, but pay attention to how many times you look back and forth between the legend and the pie. This is putting a totally unnecessary strain on your brain! Take a look at the horizontal bar chart — no jumping back and forth needed!
With a horizontal bar chart, the label sits right next to the data, and it doesn’t need to be abbreviated to do so (this is one reason that I find horizontal bar charts to be better than vertical column charts in many cases — with a horizontal orientation, the labels have more width with which to work).
3. Those Pesky Near-Zero Values
Pie charts suck at the small percentages. Small percentage categories wreak havoc on the labeling issue, for sure, but they’re also nearly impossible to compare to each other. In the example above, the smallest percentage is 3%, and that’s almost manageable. But, heaven forbid you have a couple of pesky sub-one-percent categories, and you’re looking at wedges that look suspiciously like the lines between wedges.
4. Seeing Small Differences
Fundoogles & Flibbers came in at 3%, while Dracula’s Mickety Micks came in at 5%. Do the wedge sizes really look different? That’s a fundamental challenge with pie charts — we don’t do a very good job of comparing the areas of these odd sorta-triangular-but-with-one-curved-side shapes. In the case of the bar chart, all you have to compare is lengths — much easier.
5. Economy (of Space) Is a Virtue
Check out the overall size of the charts. While they have the same font size, the same text displayed, and the same width, the bar chart is 20% shorter…and it could have been shorter still! Bar charts are more efficient space-wise. With pie charts, and largely because of the other issues listed above, it’s often necessary to make the chart larger and larger to make it readable.
Of Course, This Exampel Was At Least Flat
This post would be twice as long if I went into the additional issues of using the “3D effect” version of the pie chart.
[Update] Always Room for Improvement
Of course, the danger of posting a “here’s a better way” is that you leave yourself open for suggestions as to how the better way can be improved! See Naomi’s comment below. She raises a good point — basically, that I didn’t do a great job of heeding the data-pixel ratio with my bar chart! So, below is a revised version.
In a subsequent email exchange, Naomi made the case for keeping the x-axis and the numbers, but simply removing the “%” signs entirely and putting the word “Percent” in the axis label:
Her main point is that numbers can be read more easily if they are not cluttered with symbols like dollar signs and percent signs. And, her case for keeping the gridlines and labeled axis is that it helps show that the bars are drawn to scale — there hasn’t been any incorrect or misleading scaling (intentional or not — in the same spate of presentations that spurred this post, there was a bar chart with an accompanying table of data…and one of the bars was clearly not accurate).
I’m partial to the version with all of the lines removed, but, at this point, the debate is at a much healthier level than “pie vs. bar,” so I’m happy!