ramp <- function(values, cols, n) {
  # values: a sorted vector of values
  # col: a vector of colours corresponding to the values
  # n: total number of colours to create
  scale01 <- function(x) (x - min(x, na.rm=TRUE))/diff(range(x))
  round2 <- function(x) {
    # ensures sum of rounded values is maintained
    y <- floor(x)
    i <- tail(order(x-y), round(sum(x)) - sum(y))
    y[i] <- y[i] + 1
    y
  }
  if(is.unsorted(values)) stop('values should be sorted ascendingly', call.=FALSE)
  nv <- length(values)
  nc <- length(cols)
  stopifnot(nc==nv)

  pals <- lapply(mapply(c, cols[-nc], cols[-1], SIMPLIFY=FALSE),
                 colorRampPalette)
  nn <- round2(diff(scale01(values)*n))
  ramps <- mapply(function(p, n) p(n), pals, nn)
  unname(unlist(ramps))
}