plot.frequency.polygon <- function(n=6)
{
  data=rnorm(5000, mean=14, sd=2)
  data <- data[data>8 & data<=20]
  h<-hist(x=data, breaks=seq(8, 20, length=n+1), plot=FALSE)

  # compute the frequency polygon
  diffBreaks <- h$mids[2] - h$mids[1]
  xx <- c( h$mids[1]-(diffBreaks/2), h$mids, tail(h$mids, 1)+(diffBreaks/2) )
  yy <- c(0, h$density, 0)

  # draw the histogram
  hist(x=data, breaks=seq(8, 20, length=n+1), prob = TRUE, 
                   border="gray", col="gray90",
                   xlab="βαθμοί", ylab="σχετική συχνότητα", main="", 
                   cex.lab=1.5, cex.axis=1.5, family="serif")
  lines(xx, yy, lwd=2, col ="royalblue")
  mtext(text=as.expression(substitute("πλήθος κλάσεων"==n)), 
        col="red", family="serif", cex=1.8)
}