fMonth <- factor(airquality$Month) # Convertit la variable "Month" en facteur.  
ozone.box <- ggplot(airquality, aes(x = fMonth, y = Ozone)) + geom_boxplot()
solar.box <- ggplot(airquality, aes(x = fMonth, y = Solar.R)) + geom_boxplot()
temp.box  <- ggplot(airquality, aes(x = fMonth, y = Temp)) + geom_boxplot()
wind.box  <- ggplot(airquality, aes(x = fMonth, y = Wind)) + geom_boxplot()