You want to make a line graph with more than one line.

In addition to the variables mapped to the x- and y-axes, map another (discrete) variable to colour or linetype, as shown in Figure 4-6:

# Load plyr so we can use ddply() to create the example data set
library(plyr)
# Summarize the ToothGrowth data
tg <- ddply(ToothGrowth, c("supp", "dose"), summarise, length=mean(len))

# Map supp to colour
ggplot(tg, aes(x=dose, y=length, colour=supp)) + geom_line()

# Map supp to linetype
ggplot(tg, aes(x=dose, y=length, linetype=supp)) + geom_line()

Figure 4-6. Left: a variable mapped to colour; right: a variable mapped to linetype