Bar Graphs - R Graphics - page 27

Figure 3-4. Graph with grouped bars
Discussion
The most basic bar graphs have one categorical variable on the x-axis and one continuous vari-
able on the y-axis. Sometimes you'll want to use another categorical variable to divide up the
data, in addition to the variable on the x-axis. You can produce a grouped bar plot by map-
ping that variable to fill , which represents the fill color of the bars. You must also use posi-
tion="dodge" , which tells the bars to “dodge” each other horizontally; if you don't, you'll end
up with a stacked bar plot ( Making a Stacked Bar Graph ).
As with variables mapped to the x-axis of a bar graph, variables that are mapped to the fill color
of bars must be categorical rather than continuous variables.
To add a black outline, use colour="black" inside geom_bar() . To set the colors, you can use
scale_fill_brewer() or scale_fill_manual() . In Figure 3-5 we'll use the Pastel1 palette
from RColorBrewer :
ggplot(cabbage_exp, aes(x = Date, y = Weight, fill = Cultivar)) +
geom_bar(position = "dodge" , colour = "black" ) +
scale_fill_brewer(palette = "Pastel1" )
Other aesthetics, such as colour (the color of the outlines of the bars) or linestyle , can also
be used for grouping variables, but fill is probably what you'll want to use.