Thursday, September 15, 2011

Raising the Bar

"Let thy speech be better than silence, or be silent" - Dionysius the Elder

I was involved in a dialog recently about this post. It made me consider some things about data presentation that I've been reluctant to admit. First, not all audiences are created equal and, more importantly, there is emotion involved.

I live in a world where precision is expected and any lack of clarity is considered detrimental to a cause. For the most part I present material to an informed technical audience who is prepared to consume data related to the topic at hand. But there are often situations where a presenter doesn't have such luxuries - in fact, an audience may struggle with the topic so much that getting across high level information is all that one can hope for. In a scenario like this, one should use any means necessary (within reason) to get a point across. I'm still not convinced this is requirement enough for a pie chart but it does raise a valid point.

In my mind there is something more driving than the aptitude of an audience, however, and that is the emotional reaction they can have to your graphics. For better or worse people are emotionally attached to pie charts. Many individuals have a visceral reaction when they see one knowing they can quickly make sense of the data in front of them. Forget about accuracy - we are talking basic understanding. For me, this is harder to ignore; it opens the door to using something like a pie chart to avoid alienating your audience.

The part about this that is hard for me is that I rant about visual display; probably too much for my contribution to alternatives. I'm also critical about style - often to the point of exhaustion. I just can't seem to relinquish my position that pie charts really are a failure but the points above are nagging at me: how do you captivate audience that expects the general view without sacrificing the details? I stumbled upon an idea recently that I hope can help bridge the gap.

I was reading Crowdsourcing Graphical Perception: Using Mechanical Turk to Assess Visualization Design the other day which led me to Cleveland and McGill's original study. One test that really stood out to me was the position-angle test where subjects were exposed to a Cartesian bar graph and a pie chart each containing the same data. The subjects were tasked with estimating values of the shapes. In 40 tests, only three times was the pie chart more accurate (on average) than the bar chart.

The original study also mentions that pie charts are "one of the most commonly used graphs for showing the relative sizes of a whole." Certainly, a pie chart intuitively exposes that the whole is the sum of it's parts. In fact, I think it does so better than some of the alternatives - stacked bar charts and treemaps. It is unfortunate that we are unable to accurately decipher the actual portions of those parts. What is really needed is the ability to combine the concept of 'sum of the parts' with direct representation of data but, to the best of my knowledge, this does not exist in standalone form.

Well, I've been exploring processing more and more lately and the idea struck me to combine the two chart types in a way that allowed both versions of the data to be presented (without having to share the display real estate). I came up with an interactive pie/bar chart fusion. On the surface it appears as a standard pie chart:
But when the user clicks any of the sections, it transitions into a bar chart with details of the data while keeping a shade of the relevant pie slice in the background.
Now, I eluded to the fact that this not a complete solution; it only helps to bridge the gap. Unfortunately, this graphic relies on user interaction (mouse clicks) for the transition which pretty much excludes it for most presentations. However, as PDF now supports Javascript, online resources are becoming prevalent and users can download these open source tools on their own the availability for melding these approaches becomes tangible.

I still don't condone the use of pie charts. However, instead of just describing the problems associated with them I'm finally trying to present a solution.

You can find the code for this on github.
Actual interactive visualization can be found here.


  1. I find it ironic that an article about the quality of data presentation is presented on a webpage whose formatting makes is barely readable (text of varying dim shades of gray on a black background? Really?!?).

  2. Scott,

    I realize that for most people this choice is far from the norm but I am colorblind. It's a difficult relationship, me and colors :)

    The first time I put up a public facing webpage was when I was a TA in college and I did it in colors that I thought were pleasant. After half of the class left with bleeding eyes I decided it was time to stick to something I knew.