# Functions to graph density functions.

gexp = function(endpoints,rate) {
  require(lattice)
  if( !(is.vector(endpoints)) || (length(endpoints) != 2) )
    stop("endpoints must be a vector of length 2")
  if(endpoints[1] > endpoints[2])
    endpoints = rev(endpoints)
  x = seq(endpoints[1],endpoints[2],length=101)
  y = dexp(x,rate=rate)
  r = max(y)*0.05
  gplot = xyplot(y ~ x,
    panel=function(x,y,...) {
      panel.mathdensity(dmath=dexp,args=list(rate=rate),n=501,lwd=2,col="red")
      panel.abline(h=0)
    },
    xlab="x",
    ylab="density",
    ylim=c(-r,max(y)+r))
  print(gplot)
  return(invisible(gplot))
}

ggamma = function(endpoints,shape,rate) {
  require(lattice)
  if( !(is.vector(endpoints)) || (length(endpoints) != 2) )
    stop("endpoints must be a vector of length 2")
  if(endpoints[1] > endpoints[2])
    endpoints = rev(endpoints)
  x = seq(endpoints[1],endpoints[2],length=101)
  y = dgamma(x,shape=shape,rate=rate)
  r = max(y)*0.05
  gplot = xyplot(y ~ x,
    panel=function(x,y,...) {
      panel.mathdensity(dmath=dgamma,args=list(shape=shape,rate=rate),n=501,lwd=2,col="red")
      panel.abline(h=0)
    },
    xlab="x",
    ylab="density",
    ylim=c(-r,max(y)+r))
  print(gplot)
  return(invisible(gplot))
}

gbeta = function(endpoints,shape1,shape2) {
  require(lattice)
  if( !(is.vector(endpoints)) || (length(endpoints) != 2) )
    stop("endpoints must be a vector of length 2")
  if(endpoints[1] > endpoints[2])
    endpoints = rev(endpoints)
  if(endpoints[1]==0 && shape1<1)
    stop("the Beta density is infinite at x=0 when alpha (shape1) < 1")
  if(endpoints[2]==0 && shape2<1)
    stop("the Beta density is infinite at x=1 when beta (shape2) < 1")
  x = seq(endpoints[1],endpoints[2],length=101)
  y = dbeta(x,shape1=shape1,shape2=shape2)
  r = max(y)*0.05
  gplot = xyplot(y ~ x,
    panel=function(x,y,...) {
      panel.mathdensity(dmath=dbeta,args=list(shape1=shape1,shape2=shape2),n=501,lwd=2,col="red")
      panel.abline(h=0)
    },
    xlab="x",
    ylab="density",
    ylim=c(-r,max(y)+r))
  print(gplot)
  return(invisible(gplot))
}