labs(...)

xlab(label)

ylab(label)

ggtitle(label, subtitle = NULL)

p <- ggplot(mtcars, aes(mpg, wt)) + geom_point()
p + labs(title = "New plot title")
p + labs(x = "New x label")
p + xlab("New x label")
p + ylab("New y label")
p + ggtitle("New plot title")

# Can add a subtitle to plots with either of the following
p + ggtitle("New plot title", subtitle = "A subtitle")
p + labs(title = "New plot title", subtitle = "A subtitle")

# Can add a plot caption underneath the whole plot (for sources, notes or
# copyright), similar to the \code{sub} parameter in base R, with the
# following
p + labs(caption = "(based on data from ...)")

# This should work independently of other functions that modify the
# the scale names
p + ylab("New y label") + ylim(2, 4)
p + ylim(2, 4) + ylab("New y label")

# The labs function also modifies legend labels
p <- ggplot(mtcars, aes(mpg, wt, colour = cyl)) + geom_point()
p + labs(colour = "Cylinders")

# Can also pass in a list, if that is more convenient
p + labs(list(title = "Title", subtitle = "Subtitle", x = "X", y = "Y"))