Wrap long axis labels via labeller=label_wrap in ggplot2

The "scales" package includes a function very much like Claude's and Leonardo's: wrap_format.

library(scales)
ggplot(df, aes(x, y)) + geom_bar(stat = "identity") + 
    labs(x = "", y = "Number of Participants") + 
    scale_x_discrete(labels = wrap_format(10))

You don't need the label_wrap function. Instead use the str_wrap function from the stringr package.

You do not provide your df data frame, so I create a simple data frame, one that contains your labels. Then, apply the str_wrap function to the labels.

library(ggplot2)
library(stringr)

df = data.frame(x = c("label", "long label", "very, very long label"), 
                y = c(10, 15, 20))
df

df$newx = str_wrap(df$x, width = 10)
df

Now to apply the labels to a ggplot chart: The first chart uses the original labels; the second chart uses the modified labels; and for the third chart, the labels are modified in the call to ggplot.

ggplot(df, aes(x, y)) + 
  xlab("") + ylab("Number of Participants") +
  geom_bar(stat = "identity") 

ggplot(df, aes(newx, y)) + 
  xlab("") + ylab("Number of Participants") +
  geom_bar(stat = "identity")

ggplot(df, aes(x, y)) + 
  xlab("") + ylab("Number of Participants") +
  geom_bar(stat = "identity") +
  scale_x_discrete(labels = function(x) str_wrap(x, width = 10))

(Hopefully) improving on @Claude's answer:

get_wraper <- function(width) {
    function(x) {
        lapply(strwrap(x, width = width, simplify = FALSE), paste, collapse="\n")
    }
}

ggplot(df, aes(x, y)) + geom_bar(stat = "identity") + 
    labs(x = "", y = "Number of Participants") + 
    scale_x_discrete(labels = get_wraper(10))

Here is another way without reference to the library stringr:

ggplot(df, aes(x, y)) + 
  xlab("") + ylab("Number of Participants") +
  geom_bar(stat = "identity") +
  scale_x_discrete(labels = function(x) lapply(strwrap(x, width = 10, simplify = FALSE), paste, collapse="\n"))

where the call:

lapply(strwrap(x, width = 10, simplify = FALSE), paste, collapse="\n")

does the work of splitting the labels dynamically. The result is the same as in the first answer.