Graphics Reference
In-Depth Information
Chapter3.Bar Graphs
Bar graphs are perhaps the most commonly used kind of data visualization. They're typically
used to display numeric values (on the y-axis), for different categories (on the x-axis). For ex-
ample, a bar graph would be good for showing the prices of four different kinds of items. A bar
graph generally wouldn't be as good for showing prices over time, where time is a continuous
variable—though it can be done, as we'll see in this chapter.
There's an important distinction you should be aware of when making bar graphs: sometimes
the bar heights represent countsof cases in the data set, and sometimes they represent valuesin
the data set. Keep this distinction in mind—it can be a source of confusion since they have very
different relationships to the data, but the same term is used for both of them. In this chapter I'll
discuss this more, and present recipes for both types of bar graphs.
Making a Basic Bar Graph
Problem
You have a data frame where one column represents the xposition of each bar, and another
column represents the vertical (y) height of each bar.
Solution
Use
ggplot()
with
geom_bar(stat="identity")
and specify what variables you want on the
x- and y-axes (
Figure 3-1
):
library(gcookbook)
# For the data set
ggplot(pg_mean, aes(x
=
group, y
=
weight))
+
geom_bar(stat
=
"identity"
)
