It's become too easy and common for data visualization practitioners to point to flaws in pie charts and other artless visualizations. Far better is to pair criticism with demonstrated improvements. Kaiser Fung's junkcharts blog is the pioneer in backing words with actions, but there’s nothing stopping the rest of us from making visualization improvements. Let's all use Pi Day as motivation to clean up the data visualization world, one pie chart at a time.
Many critics have written at length against pie charts, so I'll only recap that most criticisms follow from perceptual studies showing we perceive angles and areas less accurately than positions and lengths. I won't go as far as John Tukey, the father of exploratory data analysis, who is often quoted as saying, "There is no data that can be displayed in a pie chart that cannot be displayed better in some other type of chart."
Pie charts have their supporters and can be useful for simple, low-accuracy views or when visually summing adjacent values is important, for instance. However, I think everyone can agree that pie charts fall down in many cases:
- When there are many levels.
- When the data doesn't support proportions.
- When the wedge ordering is random.
- When distorting effects are added.
These examples below from Wikipedia illustrate these pitfalls.
So what’s your Pi Day action? Look for wayward pie charts in your own work or in a public space like Wikipedia and replace them with better visualizations. Then leave a comment here or tweet with the tag #onelesspie to share your accomplishment. Anyone can edit Wikipedia, but if you're not up for editing, you can still move things forward by posting a comment on the "Talk" page for an entry or even contacting the chart author. In either case, be sure to read and follow the Five Pillars of Wikipedia.
You can use this Google image search as a starting point, but it's best if you can narrow it to your own field of expertise, such as semiconductors or genetics. That’s because you need to take a few minutes to understand the intended message of the chart in support of the text before you can improve it. Often the improvement will be a bar chart, but sometimes a table or removal may be better (Wikipedia even has a barnstar award for those "who remove unnecessary information from images or descriptions").
To get you going, here are the steps from my early start at improving the visualization of content languages for Internet websites. The page Languages used on the Internet contains two pie charts. The first one was this pie chart:
While the chart succeeds in showing that more than half of websites have English content (pie charts are good at comparisons to 50% and 25%), the rest of the chart underperforms. Plus, the percentages for this pie chart don’t add up to 100% because some websites use more than one language.
Fortunately, the data is provided in the Wikipedia page and via a link to the original source. Sometimes, you may have to hunt a little for the data. In this case, I still had do some data work because the data in the Wikipedia page was out of date.
I made a bar chart of the data and ran into one of the great problems of data visualization: handling multiple magnitudes of scale. In this case, the English usage dwarfs the others. I thought about showing the top non-English languages, but decided the English dominance was an important part of the message and left it in. To keep the emphasis on English being in more than 50% of websites, I added a label for that bar. For presentation, I added a reference line at 5% where the second-tier languages were and gave "Others" a different styling.
Once the chart was completed, I saved it as resolution-independent SVG (though PNG is fine, too) and uploaded it to Wikimedia Commons using the Upload Wizard. In the description, I added a link to the data to make it easier for the anyone who improves on my chart. Finally, I updated the referencing Languages used on the Internet page with the new image and new data.
I also left a note on the user page of the original chart author explaining my changes in case the author has objections. And I still need to update a few other pages that use the old images.
The process was not simple. I rarely edit Wikipedia and had never uploaded image files before, so it was a bit slow going. But it's nice to try to move things forward. #onelesspie