Representing Data - Data Points: Visualization That Means Something - page 118

In the deaths chart shown in Figure 3-24, from the Statistical Atlas of the United
States published in 1874, length is used to show the distribution of deaths for
each state, by age and gender. The horizontal axis on each plot represents the
number of deaths on a linear scale, and the vertical axis represents numeric
categories that represent age groups.

Figure 3-25 shows generalized combinations, which cover common visual-
ization types such as the line chart, bubble plot, and choropleth map. The
key is learning how each component fits together—how each ingredient can
complement and enhance others—to make something more useful than the
separate parts.

Now try to fit components together, starting with the data and then building
on that foundation. Figure 3-26 is a data table from the United States Census
Bureau that shows educational attainment (high school graduate or more,
bachelor's degree or more, and advanced degree or more) by state, in 1990,
2000, and 2009. The values are percentages for people 25 years old and over.
These are important bits about the data that you need to know before actually
looking at the data.

The “or more” for each column means you can't just add the values from each
column because there's overlap between them. If you want to make a pie chart
that shows the values of each column, you must do some math. For example,
the United States estimate for people with a high school degree (or equivalent)
or more is 75.2 percent. Subtract those with a bachelor's degree or more, 20.3
percent, to get rid of the “or more” part of the high school value, which gives
you 54.9 percent of people with only a high school degree.

It's also useful to know the sample population. If it were everyone in America,
the percentages would be lower, or if for some odd reason the sample was
those under 18, the percentages for an advanced degree or more would
represent a tiny group of people who skipped or advanced quickly through
elementary and high school.

So you have the most important part of any visualization: the data. There are
nine columns, spread out over 3 years and three subcategories, plus one more
column for state names, so you can visualize the data on multiple dimensions.
You might want to focus on educational attainment in 2009, in which case, a
few bar charts, as shown in Figure 3-27, could work.

FIGURE 3-24 (facing page)
Chart showing the distributions
of deaths, based on United States
census of 1870 by Francis A. Walker