R/normProb.r

In pbreheny/breheny: Miscellaneous Functions

#' Draw a normal curve and shade area under an interval (probability)
#'
#' @param a,b       Endpoints of the interval
#' @param outside   Shade the area outside (a,b)?  Default is FALSE (i.e., shade the area inside a and b)
#' @param ...       Additional arguments to `plot()`
#'
#' @examples
#' normProb(1, 2)
#' normProb(1, 2, outside=TRUE)
#' normProb(1, 2, lwd=5, col='pink', main='Nice!')
#' @export

normProb <- function(a, b, outside=FALSE, ...) {
  x <- seq(-4,4,len=101)
  y <- dnorm(x)
  plot(x, y, type="l", ylab="Density", yaxs="i", bty="n", ylim=c(0,0.5), las=1, ...)
  if (!outside) {
    x1 <- seq(a, b, len=101)
    y1 <- dnorm(x1)
    polygon(c(x1, max(x1), min(x1)), c(y1, 0, 0), col="gray80", border=NA)
  } else {
    x1 <- seq(-4,a,len=101)
    y1 <- dnorm(x1)
    polygon(c(x1,max(x1),min(x1)),c(y1,0,0),col="gray80", border=NA)
    x2 <- seq(b,4,len=101)
    y2 <- dnorm(x2)
    polygon(c(x2,max(x2),min(x2)),c(y2,0,0),col="gray80", border=NA)
  }
}