The Context of Data Visualization - Data Visualization: A Successful Design Process - page 8

On the right-hand side is a demonstration of the "Law of Proximity". The arrangement
of closely packed-together pairs of columns means we assume these to be related and
distinct from the other pairings. We don't really view this display as six columns,
rather we view them as three clusters or sets.
At the root of visual perception knowledge is the understanding that our visual
functions are extremely fast and efficient processes whereas our cognitive
processes, the act of thinking, is much slower and less efficient. How we exploit
these attributes in visualization has a significant impact on how effectively the
design will aid interpretation.
Consider the following examples, both portraying analysis of the placement of
penalties taken by soccer players.
When we look at the first image, the clarity of the display allows us to instantly
identify the football symbols, their position, and their classifying color. We don't
need to think about how to interpret it, we just do. Our thoughts, instead, are
focused on the consequence of this information: what do these patterns and insights
mean to us? If you're a goalkeeper, you'll be learning that, in general, the penalty
taker tends to place their shots to the right of the goal.
Image republished under the terms of "fair use", source: http://www.facebook.com/castrolfootball
By contrast, this second display's attempt to portray the same type of data
presentation causes significant visual clutter and confusion. Rather than using a
simple and relatively blank image like the previous one, this display includes strong
colors and imagery in the background. The result is that our eyes and brain have
to work much harder to spot the footballs and their colors because the data layer
has to compete for attention with the background imagery. We are therefore unable
to rely on the capabilities of our preattentive visual perception (determined by the
Law of Similarity) because we cannot easily perceive the shapes and their attributes
representing the data. This delays our interpretative processes considerably and
undermines the effectiveness and efficiency of the communication exchange.