Small Multiples

Summary: An effective technique for showing differences over time or among categories of data in a small amount of space.

Details

According to Edward Tufte (The Visual Display of Quantitative Information, 2nd Ed., p. 168):

Small multiples resemble the frames of a movie: a series of graphics, showing the same combination of variables, indexed by changes in another variable.

An example is “The Horse in Motion”, which shows how the gait of a running horse changes over time.

The Horse in motion. 'Sallie Gardner,' owned by Leland Stanford; running at a 1:40 gait over the Palo Alto track, 19th June 1878 — The Horse in motion. "Sallie Gardner," owned by Leland Stanford; running at a 1:40 gait over the Palo Alto track, 19th June 1878. (Source)

Another example are sparklines, which show how data change over time:

Example of sparklines from Wikipedia. — (Source)

Small multiples allow the viewer to visually compare characteristics of data quickly among categories. It is an effective technique when:

Recognizing general trends is more important than showing exact numbers
There are large numbers of categories of data to be compared
Space is limited