A discussion with a coworker the other day centered around when a field has too many unique values to make it worthwhile for any data visualizations (as is, without any groupings). The answer we left with* was that when a pie chart's pieces blend into each other, it is definitely too many. So, data viz geek that I am, I had to draw it out for my own amusement:
Oh wait, I can see where this might be confusing. Let me label it:
(Though is there an argument to be made that this pie chart may actually succeed in showing that pretty much every group is the same?)
*By the way, our answer was tentatively, 10-20 points**. But that's a topic for an entirely different post!
**Of course that doesn't mean 10-20 points on a pie cart....