Multiple line comparisons
Oct 17, 2005
An informative, and rather sophisticated, graphic appeared in the Times this weekend so lets appreciate it:
Contained in this chart were two key messages:
- while in general manufacturing employment has shrunk in most Western industrialized nations during the last 10 years, the trajectory of decline differed by country;
- while manufacturing has lost employment in the U.S., other sectors have gained.
By establishing comparability, this chart significantly betters our understanding of the U.S. manufacturing data. If only the U.S. data were available (as shown on the right), the only appropriate emotion would have been groan and moan.
To tell the full story, the chart used two related line charts: superimposed lines for the international comparison, and small multiples for the U.S. comparison by industry.
In general, superimposed lines provide better aid to visual comparison although plotting large number of lines or large number of crossings can result in an inextricable mess. Also, only so many shades of gray or dotted-ness could be used before readers get dizzy.
However, superposition saves space which is a key disadvantage of small multiples. When space is of minor concern, as in this example, small multiples can do wonders. The graphs are nicely ordered from greatest growth to biggest decline, painting a vivid picture of the differential fortunes of various economic sectors.
Printing small multiples horizontally makes it easy to compare the growth rates but harder to compare time periods. The relative importance of rate versus time comparison must be established before the design decision. This artifact is avoided in the case of superimposed lines.
The temporal perspective is sometimes important. The top chart shows that Germany's manufacturing sector has been in continuous decline since 1993 (not sure why this arbitrary year is chosen) while for most other nations, a large drop occurred around 2001. It also indicates that the U.S. has done comparatively better since mid 2003 than France, Germany and Britain so perhaps we should re-evaluate the recent brouhaha about outsourcing to China and India.
Neither the charts nor the article address the issue of what caused the drop in employment. The blame is often placed on foreign competition but surely we must consider displacement by automation, machines, Internet, software, etc.
Reference: "Proof, Near and Far, That It's Not 1950 Anymore", New York Times, Oct 15, 2005.
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