Discrete color raster map for negative and positive R values

Question

I have two data frames that I would like to display. Dfs have the same xy coordinates, and I need a single color panel with a visible discrete color scale for both dfs, as shown here. I would like the colors in colorkey to fit self-defined breaks. a more general solution that can be applied outside this example is much appreciated

The color palette of the package RdYIBu RcolorBrewer - this is what I get after.

My code is:

library(rasterVis)
ras1 <- raster(nrow=10,ncol=10) 
set.seed(1) 
ras1[] <- rchisq(df=10,n=10*10) 
ras2=ras1*(-1)/2 
s <- stack(ras1,ras2) 
Uniques <- cellStats(s,stat=unique) 
Uniques.max <- max(Uniques)
Uniques.min <- min(Uniques)
my.at <- round(seq(ceiling(Uniques.max), floor(Uniques.min), length.out= 10),0)
myColorkey <- list(at=my.at, labels=list(at=my.at)) 
levelplot(s, at=my.at, colorkey=myColorkey,par.settings=RdBuTheme())

colorkey , ? , colorkey , .

. .

.

Answer 1

. ggplot2 - , , , - . .

#Order breaks from lowest to highest
  my_at <- sort(my_at)

#Get desired core colours from brewer
  cols0 <- brewer.pal(n=length(my_at), name="RdYlBu")

#Derive desired break/legend colours from gradient of selected brewer palette
  cols1 <- colorRampPalette(cols0, space="rgb")(length(my_at))

#Convert raster to dataframe
  df <- as.data.frame(s, xy=T)
  names(df) <- c("x", "y", "Epoch1", "Epoch2")

#Melt n-band raster to long format
  dfm <- melt(df, id.vars=c("x", "y"), variable.name="epoch", value.name="value")

#Construct continuous raster plot without legend
  #Note usage of argument `values` in `scale_fill_gradientn` -
  #-since your legend breaks are not equi-spaced!!!
  #Also note usage of coord_equal()
  a  <- ggplot(data=dfm, aes(x=x, y=y)) + geom_raster(aes(fill=value)) + coord_equal()+
        facet_wrap(facets=~epoch, ncol=1) + theme_bw() + 

        scale_x_continuous(expand=c(0,0))+
        scale_y_continuous(expand=c(0,0))+
        scale_fill_gradientn(colours=cols1,
                             values=rescale(my_at),
                             limits=range(dfm$value),
                             breaks=my_at) +
        theme(legend.position="none", panel.grid=element_blank())

#Make dummy plot discrete legend whose colour breaks go along `cols1`
  df_leg <- data.frame(x=1:length(my_at), y=length(my_at):1, value=my_at)
  gg_leg <- ggplot(data=df_leg, aes(x=x, y=y)) + geom_raster(aes(fill=factor(value))) +
            scale_fill_manual(breaks=my_at, values=cols1,
                              guide=guide_legend(title="",
                                                 label.position="bottom")) +
            theme(legend.position="bottom")

#Extract discrete legend from dummy plot
  tmp <- ggplot_gtable(ggplot_build(gg_leg))
  leg <- which(sapply(tmp$grobs, function(x) x$name)=="guide-box")
  legend <- tmp$grobs[[leg]]

#Combine continuous plot of your rasters with the discrete legend
  grid.arrange(a, legend, ncol=1, heights=c(4, 0.8))