Plot a rasterVis::levelplot with a colour ramp diverging around zero

diverge0.R

diverge0 <- function(p, ramp) {
  # p: a trellis object resulting from rasterVis::levelplot
  # ramp: the name of an RColorBrewer palette (as character), a character 
  #       vector of colour names to interpolate, or a colorRampPalette.
  require(RColorBrewer)
  require(rasterVis)
  if(length(ramp)==1 && is.character(ramp) && ramp %in% 
     row.names(brewer.pal.info)) {
    ramp <- suppressWarnings(colorRampPalette(brewer.pal(11, ramp)))
  } else if(length(ramp) > 1 && is.character(ramp) && all(ramp %in% colors())) {
    ramp <- colorRampPalette(ramp)
  } else if(!is.function(ramp)) 
    stop('ramp should be either the name of a RColorBrewer palette, ', 
         'a vector of colours to be interpolated, or a colorRampPalette.')
  rng <- range(p$legend[[1]]$args$key$at)
  s <- seq(-max(abs(rng)), max(abs(rng)), len=1001)
  i <- findInterval(rng[which.min(abs(rng))], s)
  zlim <- switch(which.min(abs(rng)), `1`=i:(1000+1), `2`=1:(i+1))
  p$legend[[1]]$args$key$at <- s[zlim]
  p[[grep('^legend', names(p))]][[1]]$args$key$col <- ramp(1000)[zlim[-length(zlim)]]
  p$panel.args.common$col.regions <- ramp(1000)[zlim[-length(zlim)]]
  p
}