Understanding Data - Data Points: Visualization That Means Something - page 363

FI G U R E 1-13 A firefly in the night
by Kristian Cvecek, http://quit007
.deviantart.com/
Although each moment in flight seems like a random point in space to an
observer, a pattern emerges in Cvecek's photos, as shown in Figure 1-14. It's
as if the fireflies move along the walking path and circle around the trees with
a predetermined destination.
There is randomness, though. You can guess where a firefly goes next based on
its flight path, but how sure are you? A firefly can bolt left, right, up, and down at
any moment, and that variability, which makes each flight unique, is what makes
fireflies so fun to watch and the picture so beautiful. The path is what you care
about. The end point, start point, and average position don't mean nearly as much.
With data, you can find patterns, trends, and cycles, but it's not always (rarely, actu-
ally) a smooth path from point A to point B. Total counts, means, and other aggre-
gate measurements can be interesting, but they're only part of the story, whereas
the fluctuations in the data might be the most interesting and important part.
Between 2001 and 2010, according to the National Highway Traffic Safety
Administration, there were 363,839 fatal automobile crashes in the United States.
No doubt this total count, over one-third of a million, carries weight because it
represents the lost lives of even more than that. Place all focus on the one number,
as in Figure 1-15, and it makes you think or maybe even reflect on your own life.
However, is there anything you can learn from the data, other than that you
should drive safely? The NHTSA provides data down to individual accidents,
which includes when and where each occurred, so you can look closer.
In Figure 1-16, every fatal crash in the contiguous United States between 2001
and 2010 is mapped. Each dot represents a crash. As you might expect, there
is a higher concentration of accidents in large cities and major highways; there
are fewer accidents where there are fewer people and roads.