Look again. The point isn't that he's using blue for red and vice versa, the point is that the 3d chart distorts the information. In a 2d chart, we'd have equal shares for each color. Since it's 3d, even though blue has a smaller slice, it gets more pixels.
Pie charts, IMO, are almost never the correct choice. Even in cases where there's a logical summation to 100%, absolute values (the size of the pie altogether) is often also relevant; and pies with many slices are hard to compare, when the angles for different slice shadings are at different orientations.
Finally, too many charts rely on colour rather than patterns or markings to distinguish choices. That usually makes them hard to read for colour-blind people, as light shades of green and yellow, or dark blue and purple, etc. end up being used.
People can quantify the difference between two lines of disparate length much more easily than they can quantify the angle difference of slices of a pie chart[1]. Pie charts are evil and should never be used. A full 86.1% of this comment is fact; the rest is opinion.
Personally, I dislike pie charts for that reason, plus they're hard to scan. The text describing what area means what is floating all over, or connected by a bunch of parallel lines, or (arguably better) mostly absent / in a legend.
Bar charts, especially horizontal, have skimmable labels. Pie charts do not.
For those of us who lack graphical intuition (I'm guilty), look to Stephen Few (who looks to Mr. Tufte) for best ways to visualize business (and scientific) data.
What are you using the graph for? If you're trying to convey information, a 3d chart of any sort at all is less than effective. If nobody cares about the information and you're going through the motions of a "presentation" for merely corporate-cultural reasons, a 3d chart may add some useful bling.
Your brain doesn't see pixels. It sees volume. And it is really good at doing so. You can process 1000x the amount of information in 3D that you can in 2D. http://aqumin.blogspot.com/
This is completely dependent on the angle that it is viewed from. IMO the 3D pie chart should only be slightly angled so as to give a hint of a shadow but not as extremely tilted as in this example. If it was tilted anymore and depending on which side it was viewed from, you can make it completely red or blue.
In other words, I like the irony but this is not really a good example.
Why is the Blue label pointing to the red region and the Red label pointing to the blue region? Or are their lines actually pointing to the back of the chart where you can't see the line ends?
There are more blue pixels in the image than red. This is because of the thickness of the disk. Therefore in representing the proportion of blue pixels it's more than half the area of the circle. Therefore he has one slice which is more than half the area of the circle, and that half represents the blue pixels. He colors that red.
And vice versa.
The diagram is completely accurate, and very misleading. Adn deliberately so. It can be difficult to get pie charts to represent data accurately, and in a way that doesn't mislead the viewer.
On the other hand, correct use of pie charts can be very effective. It's just that you don't often see it.
I think his point is that when you include the pixels which represent the thickness of the disc, you inflate the apparent size of that slice.
So while you can clearly see the size of the elements, it requires thought and interpretation. Compare to a 2D flat chart, which is very clear although not so pretty. Shouldn't a visualisation require as little thought as possible?
Honestly, I always find split bars easier to read in terms of proportions than a pie. The human mind is better at estimating linear distances than angles.
Not to mention with split bars you can more easily show the proportional change of things over another axis (usually time).
I actually got the point, I was saying that it was still obvious which part was bigger, which is what a pie chart should be used for. Anything more complex than that, and it's being used incorrectly.
The whole point of a chart is at-a-glance understanding of numbers and stats that would normally take a more careful once-over.
And the whole point of a pie chart is to use one thing -- the size of a slice -- to represent a single datum.
A 3d pie chart muddles both of those things for form-over-function reasons. How is this even contentious? Pie charts are notoriously bad anyway, especially if there's not a huge difference between the data points. This makes them notably worse.
The purpose of a pie chart isn't to show which is bigger (which is much easier to convey in text), but to give relative proportions.
So could you accurately estimate the two sizes? Research suggests you'd be better off with a bar chart than with a pie chart except in certain cases (25% vs 75%) and that isometric pie charts are worse than flat ones.
It would be just as trivial to create a 3D bar chart that has the same effect as the example (accurate, self-referential, and confusing because of the color/label mismatch). While clever, this example isn't an indictment of pie charts, because it clearly shows the relative proportions of two quantities (as would a bar chart).
There's two different links in these comments that explain at length why pie charts do not clearly show relative proportion. This is even less true of 3D pies.
but we have to have more 3d. 3d is the current immersive property for a richer viewer/user experience. and this needs to expand even to pie charts. the next thing will be to add smell to the pie charts - so that they smell like pie.
The percentage of blue pixels in the chart is shown by the red section of the chart, and vice-versa (notice the labeling).
See also: http://xkcd.com/688/