In the Sunday, March 19th New York Times Magazine, accompanying an article on the decline in global conflict, was the following chart:

Apart from the odd terminology (extrastate vs. interstate? And "war between states" means something quite different here in North Carolina...), I was puzzled by the color choice. Was this a stacked area graph, or one where the areas were superimposed, as the color scheme seemed to suggest? I went to the Human Security Centre website and found the original:

It is indeed a stacked area graph. The second paragraph of the caption reads:

Figure 1.1 is a ‘stacked graph’, meaning that the number of conflicts in each category is indicated by the depth of the band of colour. The top line indicates the total number of conflicts of all types in each year. Thus in 1946 there were five extrastate conflicts, two interstate conflicts, ten intrastate conflicts, and 17 conflicts in total.

A good rule of thumb is that if you need to spell out how to read the graph, there’s a design problem. In this case, we're being misled by the “mountain illusion.” The human eye is used to distant objects being lighter in color, so sees the area chart as a mountain range, and hence layered.

The New York Times designers, 5W Infographic, exacerbated this illusion by changing the color palette to different percentages of magenta. On the plus side, they simplified the x-axis by only numbering decades, labelled each area directly instead of with a legend, removed the frame, and replaced the x-axis tick marks with pale vertical gridlines.

How to get rid of the mountain illusion? The most obvious solution is to change the color scheme by reversing the light-to-dark direction, but this doesn't seem to help much. Better is a color scheme that flattens out the illusory layering. Making the contrast color as similar as possible to the main color seems to work best.

Another option is to change the stacking order, so the smaller series sits on top of the larger, which also fights the mountain illusion. Unfortunately this makes it much harder to discern trends in the smaller series; they're swamped by the larger. Another complication here is that one series disappears, so the trick is to distinguish between the two that remain. I've used a solid line, but a dotted line would be better, and subtle changes in shade best. The final and probably superior alternative is to just unstack everything in Illustrator, by making three graphs and combining them.

While unstacking this area graph makes it unambiguous, it’s much harder to see the total conflicts for a year. Does this matter? As always, it depends on the point one is trying to make; in this case, that world conflict has declined. The trend is pretty easy to see, as the majority of the data are in just one series. If total conflicts were important, one could add a line:

References: The accompanying article was Wonderful World, by James Traub, NYT Magazine 3/19/2006. The lovely Smoky Mountains photo is by someone called Melissa, who unfortunately has no contact details, or I'd have asked. The folks at the Human Security Center at the University of British Columbia helpfully supplied the original data from their Human Security Report 2005.

(Update: charts by 5W Infographics no longer appear in the NYT magazine...)