mtcars2 <- mtcars mtcars2$trans <- factor(mtcars$am, levels=0:1, labels=c("automatic", "manual")) mtcars2$gear <- as.factor(mtcars$gear) mtcars2$am <- NULL mtcars2$vs <- NULL mtcars2$drat <- NULL mtcars2$carb <- NULL mtcars2$wt <- NULL mtcars2$hp <- NULL mtcars2$qsec <- NULL # To keep R CMD check happy mpg <- mtcars2$mpg p <- ggplot(mtcars2) p print( p )