bluered <- function(data, scale = c("equal", "linear", "split"), white = median(data), yellow = 0, cyan = 0, invert = FALSE, format = c("hex", "rgb")) {

# Generate a Colour Scale for Plotting
#
# Description:
#
#      Returns a colour scale corresponding to specified data values
#      or a specified number of colours.

# Usage:
#
#      bluered(data, scale = "equal", white = median(data), yellow = 0,
#          cyan = 0, invert = FALSE, format = "hex")
#
# Arguments:
#
#   	 data: A vector of distinct data values corresponding to the colours
#              or the number of colors (>= 1) to be created.
#
#  	 scale: Controls the colour gradients (see Details). Must be either
#         	  `"equal"' (the default), `"linear"' or `"split"' (or any
#         	  unique partial match).
#
#  	 white: The data value corresponding to the colour white.
#
# 	 yellow: A number between 0 and 1 indicating the amount of yellow to
#      	   insert between white and red. If 0 (the default) then no yellow
#         	   is used. Larger values indicate more yellow.
#
#   	 cyan: A number between 0 and 1 indicating the amount of cyan to
#            insert between white and blue. If 0 (the default) then no cyan
#            is used. Larger values indicate more cyan. Not implemented.
#
# 	 invert: Logical: if `TRUE', negative data values are red and positive
#              values are blue; if `FALSE' (the default) then vice-versa.
#
# 	 format: The format in which the colours are returned (see Value). Must
#         	   be either `"hex"' (the default) or `"rgb"' (or any unique
#         	   partial match).
#
# Details:
#
#      One colour is created for each value in `data'. Colours range from
#      blue for data values less than `white' to red for data values
#      greater than `white'. If `scale' equals `"linear"' then colour
#      intensity scales linearly with absolute data value: highest
#      intensity corresponding to the maximum absolute value of `data'.
#      If `scale' equals `"split"' then linear intensity scales are
#      constructed separately for blue and red colours so that the
#      maximum intensity is attained by both blue and red. If `scale'
#      equals `"equal"' then intensities are equally spaced regardless of
#      the data values.
#
# Value:
#
#      If `format' is `"hex"' then returns a character vector of
#      hexadecimal RGB numbers. If `format' is `"rgb"' then returns a
#      matrix with three columns containing RGB intensities.
#
# See Also:
#
#      `col2rgb', `colours', `rgb'
#
# Author:
#
#      Chris Ferro <c.a.t.ferro@reading.ac.uk> 4 May 2005
#
# Examples:
#
#      image(volcano, col = bluered(seq(100, 200, 10), "linear"))
#      image(volcano, col = bluered(11))
#      image(volcano, col = bluered(11,yellow=1))


  if (length(data)==1){data<-seq(0,data-1,by=1)}
  scale <- match.arg(scale)
  format <- match.arg(format)
  data <- unique(data) - white
  if(invert) data <- -data
  data <- sort(data)
  nb <- sum(data < 0)
  nr <- sum(data > 0)
  n <- length(data)
  if(scale == "equal") {
    if(nb == 0) r <- rep(1, n)
    else r <- pmin(1:n - 1, nb) / nb
    if(nr == 0) b <- rep(1, n)
    else b <- pmin(n - 1:n, nr) / nr
  } else if(scale == "linear") {
    dmax <- max(abs(data))
    if(nb == 0) r <- rep(1, n)
    else r <- pmin(data + dmax, dmax) / dmax
    if(nr == 0) b <- rep(1, n)
    else b <- pmin(dmax - data, dmax) / dmax
  } else {
    if(nb == 0) r <- rep(1, n)
    else r <- pmax(data[1] - data, data[1]) / data[1]
    if(nr == 0) b <- rep(1, n)
    else b <- pmin(data[n] - data, data[n]) / data[n]
  }
  g <- pmin(r, b)
  y <- min(max(yellow, 0), 1)
  g[b < 1] <- 1 - (1 - y) * (1 - b[b < 1]) - y * (1 - b[b < 1])^2
  b <- 1 - (1 + y) * (1 - b) + y * (1 - b)^2
  b[b < 1] <- b[b < 1] * (1 - y) 
#  c <- min(max(cyan, 0), 1)
#  g[r < 1] <- 1 - (1 - c) * (1 - r[r < 1]) - c * (1 - r[r < 1])^2
#  r <- 1 - (1 + c) * (1 - r) + c * (1 - r)^2
  if(invert) {
    r <- rev(r)
    g <- rev(g)
    b <- rev(b)
  }
  if(format == "hex") return(rgb(r, g, b))
  cbind(r, g, b)
}