R - ggplot - Rounded extremity for geom_bar

问题:

This is a simple example of what my barplot look like :

x <- data.frame(aa=c(0.2,0.6,0.1), dd = c(1,2,3))
x <- melt(x, "dd")
y <- data.frame(bb=c(0.4,0.5), dd = c(1,2))
y <- melt(y, "dd")
z <- data.frame(cc=c(0.5,0.25,0.1,0.05), dd = c(1,2,3,4))
z <- melt(z, "dd")

x=rbind(x,y,z)

col=c("white","grey","blue","white","red","white","green","blue","green")
ggplot(x, aes(x = variable, y = value)) + geom_bar(stat = "identity", fill = col)

I'm wondering if there is a way to have rounded ends for my bars, like with the lineend option for a line (http://sape.inf.usi.ch/quick-reference/ggplot2/lineend) ?