###
###
###  Histograms
###
###
##################################################



#--- One Historgram with two colors
  x <- rnorm(1000)
  h <- hist(x, breaks=50, plot=FALSE)
  cuts <- (h$breaks < rep(-1,length(h$breaks)))
  plot(h, col=ifelse(cuts, "red", "gray"))
  abline(v=-1, col="blue", lwd=2)




#--- Two Historgram overlayed with opacity
  library(scales)

  x <- rnorm(1000)
  y <- rnorm(1000, 3, 2)
  h1 <- hist(x, breaks=50, plot=FALSE)
  h2 <- hist(y, breaks=50, plot=FALSE)
  plot(h1, col=alpha("blue", .9), xlim=c(-5,5))
  lines(h2, col=alpha("red", .5))



#--- Two Histogram using ggplot2
  D <- data.frame(
    gender=factor(rep(c("F", "M"), each=200)),
    weight=round(c(rnorm(200, 55, 5), rnorm(200, 65, 5)))
  )
  head(D)

  library(ggplot2)
  ggplot(D, aes(x=weight, fill=sex, color=sex)) +
      geom_histogram(position="identity", alpha=0.5)