add labels to facet_wrap boxplot - r

Question

I'm trying to modify the facet labels in the plot created in this post.
To repeat the code:
library(ggplot2)
mymatrix1 <- matrix(data = 0, nrow = 105, ncol =2)
mymatrix2 <- matrix(data = 0, nrow = 108, ncol =2)
mymatrix3 <- matrix(data = 0, nrow = 112, ncol =2)
mymatrix1[,1] <- sample(0:1, 105, replace= TRUE)
mymatrix1[,2] <- rnorm(105, 52, 2)
mymatrix2[,1] <- sample(0:1, 108, replace= TRUE)
mymatrix2[,2] <- rnorm(108, 60, 2)
mymatrix3[,1] <- sample(0:1, 112, replace= TRUE)
mymatrix3[,2] <- rnorm(112, 70, 2)
mydata <- list(mymatrix1, mymatrix2,mymatrix3)
for(i in 1:3){
mydata[[i]] <- cbind(mydata[[i]], i)
colnames(mydata[[i]]) <- c("class", "readcount", "group")
}
mydata <- as.data.frame(do.call(rbind, mydata))
## fixed scales
p <- qplot(class, readcount, data = mydata, geom="boxplot", fill = factor(class)) +
geom_jitter(position=position_jitter(w=0.1,h=0.1)) +
scale_x_continuous(breaks=c(0,1), labels=c("0", "1")) +
facet_wrap(~ group)
## free scales
p + facet_wrap(~ group, scales = "free")
My goal, in this plot, is to change the facet labels 1, 2 and 3 to something like "a" "b" or "c."
I thought that simply changing the data frame column "group" to letters instead of numbers in the for loop:
for(i in 1:3){
mydata[[i]] <- cbind(mydata[[i]], letters[i])
colnames(mydata[[i]]) <- c("class", "readcount", "group")
}
would work, but I get this error:
Error: Discrete value supplied to continuous scale.
Any suggestions? I looked at the other posts for changing facet labels, but they didn't seem to work for this type of plot (at least for me).
Thanks so much.

Answer 1

Are you looking for something like this?
mydata$group <- as.factor(mydata$group)
levels(mydata$group) <- c("A","B","C")