plotDataPoints <- function (X, idx, K, addto = TRUE) {
  
  #   plots data points in X, coloring them so that those with the same
  #   index assignments in idx have the same color
  # 
  #   plotDataPoints(X, idx, K) plots data points in X, coloring them so that those
  #   with the same index assignments in idx have the same color
  #
  
  if (addto) {
    points(X, col = rainbow(K + 1)[idx], lwd = 1.3)
  } else {
    plot(X, col = rainbow(K + 1)[idx], lwd = 1.3, type = "p")
  }
}