Today's chart comes from Pew Research Center, and the big question is why the colors?
The data show the age distributions of people who believe different religions. It's a stacked bar chart, in which the ages have been grouped into the young (under 15), the old (60 plus) and everyone else. Five religions are afforded their own bars while "folk" religions are grouped as one, and so have "other" religions. There is even a bar for the unaffiliated. "World" presumably is the aggregate of all the other bars, weighted by the popularity of each religion group.
So far so good. But what is it that demands 9 colors, and 27 total shades? In other words, one shade for every data point on this chart.
Here is a more restrained view:
Let's follow the designer's various decisions. The choice of those age groups indicates that the story is really happening at the "margins": Muslims and Hindus have higher proportions of younger followers while Jews and Buddhists have higher concentrations of older followers.
Therein lies the problem. Because of the lengths, their central locations, and the tints, the middle section of each bar is the most eye-catching: the reader is glancing at the wrong part of the chart.
So, let me fix this by re-ordering the three panels:
Is there really a need to draw those gray bars? The middle age group (grab-all) only exists to assure readers that everyone who's supposed to be included has been included. Why plot it?
The above chart says "trust me, what isn't drawn here constitutes the remaining population, and the whole adds to 100%."
Another issue of these charts, exacerbated by inflexible software defaults, is the forced choice of imbuing one variable with a super status above the others. In the Pew chart, the rows are ordered by decreasing proportion of the young age group, except for the "everyone" group pinned as the bottom row. Therefore, the green bars (old age group) are not in a particular order, its pattern much harder to comprehend.
In the final version, I break the need to keep bars of the same religion on the same row:
Five colors are used. Three of them are used to cluster similar religions: Muslims and Hindus (in blue) have higher proportions of the young compared to the world average (gray) while the religions painted in green have higher proportions of the old. Christians (in orange) are unusual in that the proportions are higher than average in both young and old age groups. Everyone and unaffiliated are given separate colors.
The colors here serve two purposes: connecting the two panels, and revealing the cluster structure.