You want to group bars together by a second variable.

Map a variable to fill, and use geom_bar(position="dodge").

In this example we’ll use the cabbage_exp data set, which has two categorical variables, Cultivar and Date, and one continuous variable, Weight:

library(gcookbook) # For the data set
cabbage_exp

 Cultivar Date Weight
      c39  d16   3.18
      c39  d20   2.80
      c39  d21   2.74
      c52  d16   2.26
      c52  d20   3.11
      c52  d21   1.47

We’ll map Date to the x position and map Cultivar to the fill color (Figure 3-4):

ggplot(cabbage_exp, aes(x=Date, y=Weight, fill=Cultivar)) +
    geom_bar(position="dodge")

Figure 3-4. Graph with grouped bars