Visualizing Relationships - Data Points: Visualization That Means Something - page 201

A skew to the left means most of your data is clustered in the lower side of
the full range. A skew to the right means the opposite. A flat line means a
uniform distribution, whereas the classic bell curve shows a clustering at
the mean and a gradual decrease in both directions.
Next take a look at a classic plot, mainly to get a feel for distribution, and
then move on to the more practical histogram and density plot.
Old School Distribution
In the 1970s, when computers weren't so popular, most data graphics
were drawn by hand. Some of the tips offered by famed statistician John
Tukey, in his topic Exploratory Data Analysis , were centered around using
pen and pencil to vary darkness of lines and shades. You could also use
hash patterns as a fill to differentiate between variables.
The stem-and-leaf plot, or stemplot, was designed in a similar manner.
All you have to do is write the numbers using an ordered method, and the
end result is a rough view of the distribution. The method was particularly
popular in the 1980s (when using statistical graphics for analysis was
gaining momentum), because it was easy to include the graphic—even if
you were writing with a typewriter.
Although there are easier and faster ways to look at distributions today,
it's useful to take a look because you can still apply the same principles
making a stem-and-leaf plot as you would a histogram.
CrEATE A STEM-AND-LEAF
If you want to live on the wild side, you can draw a stem-and-leaf plot
with pen and paper, but you can make one much quicker in R. Figure 6-21
shows a stem-and-leaf plot for worldwide birth rates in 2008, as esti-
mated by the World Bank.
As you can see, it's basic. Base numbers are on the left and trailing num-
bers are on the right. In this case, the decimal point is at the bar (|), so the
interval with the most countries in it is the one that ranges from 10 to 12
live births per 1,000 population. There's also one country, Niger, with a
birth rate between 52 and 54.