Put whisker ends on boxplot

I would like to put perpendicular lines at the ends of the whiskers like the boxplot function automatically gives.

As hinted but not implemented by @Roland, you can use stat_boxplot to implement this. The trick calling _boxplot twice and is to set the geom to errorbar for one of the calls.

Note that as R uses a pen and paper approach it is advisable to implement the error bars first the draw the traditional boxplot over the top.

Using @Roland's dummy data df

ggplot(df, aes(x=cond, y = value))  + 
 stat_boxplot(geom ='errorbar') + 
 geom_boxplot() # shorthand for  stat_boxplot(geom='boxplot')

The help for stat_boxplot (?stat_boxplot) detail the various values computed and saved in a data.frame

To resize the whiskers lines we can use the argument width = 0.5 inside the function: stat_boxplot

set.seed(42)
df <- data.frame(cond = factor(rep(c("A", "B"), each = 500)), 
                 value = c(rnorm(500, mean = 1, sd = 0.2), 
                           rnorm(500, mean = 1.5, sd = 0.1))) 

library(ggplot2)
ggplot(df, aes(x = cond, y = value)) +
       stat_boxplot(geom = "errorbar", width = 0.5) +  
       geom_boxplot() 
       

It might be possible to use stat_boxplot to calculate the whisker ends, but I am not enough of a ggplot2 wizard, so I use the base function for that.

set.seed(42)
df <- data.frame(cond = factor( rep(c("A","B"), each=500) ), 
                 value = c(rnorm(500,mean=1,sd=0.2),rnorm(500, mean=1.5,sd=0.1)))


whisk <- function(df,cond_col=1,val_col=2) {
  require(reshape2)
  condname <- names(df)[cond_col]
  names(df)[cond_col] <- "cond" 
  names(df)[val_col] <- "value"
  b <- boxplot(value~cond,data=df,plot=FALSE)
  df2 <- cbind(as.data.frame(b$stats),c("min","lq","m","uq","max"))
  names(df2) <- c(levels(df$cond),"pos")
  df2 <- melt(df2,id="pos",variable.name="cond")
  df2 <- dcast(df2,cond~pos)  
  names(df2)[1] <- condname
  df2
}



library(ggplot2)

plot1 <- ggplot(df, aes(x=cond)) 
plot1 <- plot1 + geom_errorbar(aes(ymin=min,ymax=max),data=whisk(df),width = 0.5)
plot1 <- plot1 + geom_boxplot(aes(y=value))
plot1