You have a data frame where one column represents the x position of each bar, and another column represents the vertical (y) height of each bar.

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")

Figure 3-1. Bar graph of values (with stat="identity”) with a discrete x-axis

When x is a continuous (or numeric) variable, the bars behave a little differently. Instead of having one bar at each actual x value, there is one bar at each possible x value between the minimum and the maximum, as in Figure 3-2. You can convert the continuous variable to a discrete variable by using factor():