Here's an experiment: Get a random set of pie charts without labels on them. Get a large group of people. Now get those people to go through the pie charts and add what % of the pie chart they think each slice represents.

Now, get a set of bar charts and do the same thing.

In both experiments time the participants. You'll need two groups, a group that knows they're being timed and one that doesn't know.

What group was more accurate? What group was able to complete the exercise faster?

Here's a side by side of the same information in these two different forms: http://speakingppt.com/wp-content/uploads/2012/10/stephen-fe...

The reason that using pie charts is, in and of itself, a dark pattern is because the results of this experiment support bar charts as a more concise, more easily understood way of communicating information. In other words, the only real reason you'd use a pie chart is to obfuscate information. Or, well... to lie. If you're just using it because you don't know better, then you're negligent and misleading people without even knowing it.

"then you're negligent and misleading people without even knowing it."

Is that their fault? I don't entirely think so. You might be able to claim they should have done the research before choosing that type of graph but the majority of people are not even going to give pie charts a second thought.

If there was an active campaign against pie charts and more discussion I might find fault in that persons ignorance to the matter.