I am trying to plot two histograms using histbackback and then adding color to each of the samples. The code I have seen on the R graph gallery suggests the following method of using histbackback require(Hmisc) age <- rnorm(1000,50,10) sex <- sample(c('female','male'),1000,TRUE) out <- histbackback(split(age, sex), probability=TRUE, xlim=c(-.06,.06), main = 'Back to Back Histogram') #! just adding color barplot(-out$left, col="red" , horiz=TRUE, space=0, add=TRUE, axes=FALSE) barplot(out$right, col="blue", horiz=TRUE, space=0, add=TRUE, axes=FALSE) I get the following message when I execute the snippet to add color Warning messages: 1: parameter "add" could not be set in high-level plot() function 2: parameter "add" could not be set in high-level plot() function Any help would be appreciated. [[alternative HTML version deleted]]