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.