You want to make a histogram.

Use geom_histogram() and map a continuous variable to x (Figure 6-1):

ggplot(faithful, aes(x=waiting)) + geom_histogram()

Figure 6-1. A basic histogram

All geom_histogram() requires is one column from a data frame or a single vector of data. For this example we’ll use the faithful data set, which contains data about the Old Faithful geyser in two columns: eruptions, which is the length of each eruption, and waiting, which is the length of time to the next eruption. We’ll only use the waiting column in this example:

faithful

eruptions waiting
    3.600      79
    1.800      54
    3.333      74
...