February 6, 2012
Pie charts aren't always evil. A confession of a hypocrite.
Let me set the record straight. I HATE pie charts. I’ve written about and critiqued them many, many times. But I must confess…I used them in a presentation to a customer last week.
Yes, I am a hypocrite and I realized that immediate after I created them. So why did I use them? We had a meeting two weeks ago in which people in my same role shared event optimization success stories for their customers. We were presented some compelling results that included what I found at the time to be a straight forward pie chart used in the proper context.
For a little context, Stephen Few says in his article “Save the Pies for Dessert”:
Pie charts are not without their strengths. The primary strength of a pie chart is the fact that the message “part-to-whole relationship” is built right into it in an obvious way.
Stephen continues by declaring how bar charts can accomplish this a well.
A bar graph doesn’t have this obvious purpose built into its design. Not as directly, anyway, but it can be built into bar graphs in a way that prompts people to think in terms of a whole and its parts. This can be accomplished in part by using a percentage scale.
Let’s take a deeper dive into what I created.
The primary message I wanted to communicate was the year over year contribution of each promotional type to the total sales. Why do I think this works?
- The parts-to-whole relationship is intuitive
- There are only three colors, which makes remembering what they represent easy
- The slices are in order by promotional depth (i.e., the depth of the discount)
- You can quickly scan your eyes left to right to make the comparisons
According to the buyer I was presenting to, this was easy to understand and clearly showed that promotional depth has shifted from P2 to P1, indicating less frequent deep discounts.
But then I got thinking, how else could I have presented this data. Below are several alternatives starting tart with a simple bar chart.
For me, this layout makes the year over year comparisons more difficult. I feel like I get distracted by the bars that are in between the bars I want to compare.
Perhaps reorganizing the bars so that they are year over year within each promo level will help.
This layout works well. The year over year comparisons are much quicker in this format. However, I’ve lost the parts-to-whole relationship.
Maybe making the bars side by side horizontally will help.
The side by side bars make for easy year over year comparisons. But this layout struggles to easily communicate the contribution of each promo level to the yearly total without much explanation.
Line charts communicate year over year well. Maybe those will work.
I created two line chart options, one with labels and one without. Yes, the year over year comparisons are incredibly simple, but I hate them both for two reasons.
- The parts to whole relationship is totally missing, which is core to the message I’m trying to communicate
- There is way too much white space
That leaves us with two more options. First, an area chart.
This area chart might be a good alternative because it combines both a line chart and a stacked bar. It’s too geometric for me though.
We’re left with one final option…a stacked bar chart.
A stacked bar is the obvious competitor to the pie chart. Honestly, I wish I had chosen this chart type. It communicates all of the same information as the pie chart, but in a cleaner layout. The year over year comparisons are a bit easier because the colors are closer together.
Can I promise I’ll never use pie charts again? Never is too strong of a word. But after going through this exercise and looking at all of the alternatives available, I’m confident that I’ll be relying on my old friend the stacked bar chart.