The purpose of charts is normally to make information easier, not harder, to understand

In a presentation a few years ago, there was a pie chart, but not just any pie chart, but a pie chart that appeared to be specifically designed to convey no information whatsoever. (The presenter didn't realize this, of course, and probably thought it was awesomely cool.)

The pie chart consisted of five wedges, each of which was a slightly different shade of green. No, wait, that's not right. Each wedge was a gradient from a common shade of green to a slightly different shade of green for each wedge. Accompanying the chart was a legend that described what each shade of green represented. It was completely useless. (Inspired by this story, my friend :: Wendy :: created her own monochromatic pie chart.)

If you're going to make a chart with a legend, then the items labelled by the legend should be colors which are unlikely to be confused with each other. And for goodness' sake, don't make them all subtly different shades of the same base color. (For bonus points, consider the color-blind members of your audience. And for double bonus points, also consider the blind members of your audience. But that's another topic for another day.)

Comments (23)
  1. George Jansen says:

    There is an excellent monochromatic pie chart in Column 11 of Jon Bentley’s More Programming Pearls, one that might’ve pleased Henry Ford. The ACM Portal has it available for those with accounts.

  2. Adrian says:

    Sounds like someone who didn’t quite understand an Edward Tufte lecture.  Tufte makes a point of using the "smallest, effective difference".  So rather than a rainbow of garish colors in the pie chart, he’d suggest using a palette with less diversity.  (Actually, he’d probably tell you pie charts are junk.)  Subtler differences between the wedge colors can make for a more pleasing chart, but they still need to be different enough to be "effective".  (And forget the stupid legend: Label the wedges directly!)

    Of course, even Tufte takes things too far for my tastes.  He showed an interesting map of ocean depths using a rainbow of colors.  Then he replaced the palette with a range of blue shades.  It was much subtler and easy to look at, but it completely obscured a set of undulations that had caught my eye in the original chart.  And there was no way to find the measured depth by looking at the shades in the legend.  Surrounding a patch of color with other shades of the same color influences your perception, making it difficult to impossible to match up the absolute shade to something in a legend.

  3. At the risk of spoiling an otherwise interesting whine, I should point out that the colors are generally not necessary; the wedges are usually arranged clockwise in descending order of size, with the largest wedge being just to the right of "North".  (The "Other" or "Uncategorized" wedge is always last, even if there are smaller wedges.)

    Likewise, the legend is usually arranged in the same order as the wedges.

    So unless there are a prohibitively large number of wedges, you don’t need the colors to read the graphs at all…

    Finally, I should close by saying legends (whether color-coordinated or parallel-ordered) are generally inferior to putting the name of the wedge directly on the wedge itself.

  4. Jon Peltier says:

    "The purpose of charts is normally to make information easier, not harder, to understand"

    Not always. More often the purpose of a chart is to:

    (a) Make the presenter look smart, or at least cool.

    (b) Make the presenter’s product look better than the competition.

    (c) Accentuate the positive and obfuscate the negative.

    (d) Hide the fact that the presenter doesn’t know what’s going on.

  5. Ulric says:

    right, perhaps the presenter in fact obfuscated the chart on purpose.  I’ve been on the lookout of this since I discovered there is a book out there called "How To Lie With Charts"

  6. Jim says:

    About nine years ago, I caught someone using our brand new color laser printer to print Japanese flags.

    "What are these for?"

    — "They’re pie charts."

    "They’re all red."

    — "It’s 100%."

  7. steveg says:

    Someone should buy him a copy of The Visual Display of Quantitative Information for his birthday. It’s quite cool (if you’re into that sort of thing). It’s a train timetable on the cover, btw.

  8. Ray Trent says:

    As previously noted, I think you grossly overestimate the benevolence of average presenters when it comes to making information easy to understand.

    However, your point about color-blind people would lead to the conclusion that the ideal pie-chart would be *greyscale* (hopefully with sufficient variation to allow distinguishing the slices) — which undermines your point rather than reinforcing it.

    I.e., an all-green pie chart with different green values would be *easier* for a red-green color-blind person to read than one which had red and green  (and maybe even purple and orange) slices of similar "darkness".

    There are many kinds of color-blindness, but none that I know of cause the viewer to be unable to distinguish color value.

  9. AK Wong says:

    Jim, that is the funniest thing I’ve read all week.

  10. Andrew Cook says:

    Someone needs to get in touch with the Office team then.

    In PowerPoint 2007, if you insert a chart or some SmartArt, for the one column of polychromatic Quick Styles, there are seven monochromatic ones, each solely tints of a different accent color and white.

  11. Jon Peltier says:

    Ray T – Did you read Raymond’s explanation? It wasn’t several distinct shades of green for each wedge, it was a green-to-green gradient in each wedge. Any gradient is going to hinder comprehension, and trying to match gradients on a pie with gradients in a tiny square in the legend will be impossible. Probably a bar chart is more effective, because you don’t need to try to distinguish values by color.

  12. Joe says:

    In response to earlier comments, and pie charts, Tufte comments in "The Visual Display of Quantitative Information" (paraphrasing): The only thing worse than a silly pie chart is several of them.

    It can be well argued that a pie chart will never show information in an optimal way.  Use a table or a column chart.  With a column chart all the colors can be the same and it wouldn’t matter.

  13. Jim says:

    This is doubly ironic when you factor in the team that you’re on.

  14. Dave says:

    "It’s 100%."

    I’ve seen that used seriously in "Things that make us stupid",, there’s a monochromatic pie chart of the response to a user survey labelled "No, 100%" (do a search for "100%" to find it).

    (Well, I think he’s serious, the author likes to drop odd stuff into his presentations).

  15. Nah yah says:

    On the plus side, the all green pie charts will not only save us from climate change, they will end world hunger as well!

  16. Marcus says:

    Thanks for thinking about colour blind people! It happens quite often to me that people choose green and red as their colours of choice, and I really cant distinct them exactly if they are close to each other :(

  17. Phylyp says:

    For work-related charts: Also try and make them laser printer-friendly – (Tip: try combining the colour with a pattern)

  18. Karellen says:

    You also shouldn’t do pie charts in pseudo-3D. The perspective distortion alone makes it hard for the audience to gauge the relative sizes of the segments.

    If you absolutely must, only use a slight angle from "straight down", not a really big angle as in that particularly bad linked-to example.

    As others have pointed out, if your intention is to distort the data and confuse your audience, then 3D may be a good way to achieve that.

  19. JamesW says:

    Percentage of Chart Which Resembles Pac-Man:

  20. Kip says:

    Perhaps.. the person who made the chart was color-blind, and that is just how pie charts have looked to him all his life so he didn’t know to make them any different!

  21. spirux says:

    The original pie chart is this. I would argue, that its beauty renders the legend unnecessary.

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