Pictures are great. People understand them. When someone sees a picture of a house, they are likely to recognize it as just that: a house. Similarly, when someone sees a picture of a small house next to a large house, they will immediately determine some kind of proportional relationship between the two houses. This is one reason why we see pictorial charts in books, magazines and newspapers (oh, right, and on the internet too). Robert Harris, in Information Graphics, points out a number of other benefits of pictorial charts:
- To make the document more interesting and appealing.
- To make the material more understandable to a greater number of people [they are not language-dependent].
- To improve communication in situations where the appearance of an item is better known than the name.
- To facilitate easier reading of a chart or graph by including information to orient the reader that otherwise might have been shown in a legend or note.
I can get behind these points (though, it should be said that if a chart is not needed, it probably should not be added simply to “make the document more interesting and appealing”). Pictures may be understood faster than bar charts, where the reader must trace the horizontal and vertical axes to their respective labels in order to determine what (and how much) is being represented.
That doesn’t mean we should always use pictures though. In fact, we should be careful with them. A picture can be too specific or too general, inadvertently leaving out some part of the dataset in the mind of the reader. (Who is this guy, anyway? I don’t look like that guy!). Pictures and icons can also be tricky because of different accepted traditions and conventions. A good rule of thumb might be: if you aren’t sure the picture clearly represents your data set, don’t use it.
One particularly popular—yet often misleading—form of pictorial chart is the proportional chart or diagram. Here are some examples… and thoughts on how we could do better.
Oh, my. What do we have here? From Arkin’s Graphs: How to Make and Use Them (1936), these are… well, oranges. The orange on the left represents “44,319 carloads of oranges”; the orange on the right represents “84,944 carloads of oranges”. I wonder if it the US Department of Commerce retained a single car for 10 years explicitly to facilitate this orangey comparison. That seems likely enough.
But in all seriousness, this is a pictorial proportional diagram that isn’t entirely necessary (except as an example of what not to do, as the author has used it). Generally, readers cannot estimate volume based on a diagram nearly as well as they can other visual variables. What about you? When you first saw this, did you look at it and think, “Golly, they sold 1.9 times more carloads of oranges in 1931!”? Or did you simply think, “Hmm… more oranges 10 years later.”
These ladies make the orange chart look like a work of information visualization genius. Arkin stands behind this one, suggesting that it is a stronger graphic since the volume of the figures is used to make the comparison and the pictures are representative of the data they depict (that is, the appearance of the women varies based on age bracket). Sexism of the trans-generational shopping bag aside, charts of this type have another problem to contend with. If volume of a simple shape is difficult to ascertain, deducing it from a complex or irregular shape is nearly impossible. Schmid (Handbook of Graphic Presentation guy) lists a number of common shapes which are particularly difficult to read in this context: humans (d’oh!), ships, automobiles, houses (whoops!) and domestic animals.
Our old buddy, Brinton weighs in on this topic too. He explains the problem with a specific method: “Charts of this kind with men represented in different sizes are usually so drawn that the data are represented by the height of the man. Such charts are misleading because the area of the pictured man increases more rapidly than his height. Considering the years 1696-1700, the pictured minister has about two and one half times the height of the man representing public service. The minister looks over important because he has an area of more than six times that of the man drawn to represent public service. This kind of graphic work has little real value.” Right on, Brinton.
A reoccurring theme in all of the pieces I’ve read on these charts is that: a. they are really easy to read, but b. they are often accidentally misleading, so c. the shapes used should be as uncomplicated as possible. It makes me think… you know what shape is really uncomplicated? A bar… you know, as in “bar chart”. Ha. Nothing is perfect.
That said, some intriguing alternatives to these somewhat awkward proportional charts have gained popularity of late. Some of you may remember one viral graphic from 2009 showing what $1 trillion looks like. While I’m sure there are arguments against using 2.5D space in this graphic (oblique pie charts are the worst, right!?), I actually find that it emphasizes the point. It gives the reader the ability to say, “Gosh! That money is as far as the eye can see!” Perhaps more to the point, if the author attempted creating this graphic in 2D, there would be some major readability issues due to scale. Imagine all of those palettes stacked vertically in one row. Yikes.
After all, the pictorial chart is not unlike most charts, graphs and maps. Compromises need to be made. As long as those compromises are managed well and the readers are aware of them, they are generally quite effective.
ps. I do enjoy this planet and star size comparison video.
pps. Odd timing: immediately after posting this, I saw that information aesthetics had a moderately related post today entitled Social Compare: Visually Compare the Size of Objects and Concepts. Check it out.
ppps. Bonus chart, from Manual of Charting (1924). Hogs, Rubber and Houses. Veritable peas in a roaring 20s pod.