using layout to plot rasters with a common legend - r

Question

Follow-up to this question. I am trying to plot 4 different rasters with a common scale (legend). I have managed to do it but now I want to plot them in the same graphic device, with the scale showing just once, at the right side of the device, and another legend for some elements I have plotted over the rasters placed at the bottom of the device.
I was sure this would be no problem with layout(), but turns out there is some incompatibility between the methods in plot.rasterand layout. Following the answer to the linked question, I plotted my rasters using image instead of plot, which seems to fix the problem. In this way, I was able to use layout() to get the points legend below my four rasters. However, now I can't seem to find a way of plotting just the scale to the right of my rasters images.
Example code:
library(raster)
# generate example rasters
set.seed(123)
ras1 <- raster(ncol = 10, nrow= 10)
values(ras1) <- runif(100, 1, 10)
ras2 <- raster(ncol = 10, nrow = 10)
values(ras2) <- runif(100, 5, 50)
ras3 <- raster(ncol = 10, nrow = 10)
values(ras3) <- runif(100, 10, 100)
ras4 <- raster(ncol = 10, nrow = 10)
values(ras4) <- runif(100, 8, 80)
# stack them
rasStack <- stack(ras1, ras2, ras3, ras4)
# obtain max and min values
maxv <- max(maxValue(rasStack))+1
minv <- min(minValue(rasStack))
# set the breaks between min and max values
brks <- seq(minv,maxv,by=0.1)
nbrks <- length(brks)-1
r.range <- c(minv, maxv)
# generate palette
colfunc<-colorRampPalette(c("springgreen", "royalblue", "yellow", "red"))
# add up the 4 layers for the common legend
rasTot <- ras1 + ras2 + ras3 + ras4
# plot in a loop with a common legend, using legend.only = T
par(mfrow=c(2,2))
for(i in seq_len(nlayers(rasStack))){
tmp <- rasStack[[i]]
plot(tmp, breaks=brks,col=colfunc(nbrks), legend = F, zlim=c(minv,maxv),
main = names(tmp))
plot(rasTot, legend.only=TRUE, col=colfunc(nbrks),
legend.width=1, legend.shrink=0.75,
legend.args=list(text='value', side=4, font=2, line=2.5, cex=0.8))
points(x = rnorm(5, 0, 100), y = rnorm(5, 0, 10), col = seq(1:10), pch = 16)
}
# so far so good. But when trying to add legends...
# setting layout parameters
m <- matrix(c(1,2,6,3,4,6,5,5,5),nrow = 3,ncol = 3,byrow = TRUE)
png("example_in_png_device.png")
layout(mat = m,heights = c(0.46,0.46,0.08), widths = c(0.45, 0.45, 0.1))
par(mar=c(4,2,4,2))
# plotting the images
for(i in seq_len(nlayers(rasStack))){
tmp <- rasStack[[i]]
image(tmp, breaks=brks,col=colfunc(nbrks), zlim=c(minv,maxv),
main = names(tmp))
points(x = rnorm(5, 0, 100), y = rnorm(5, 0, 10), col = seq(1:10), pch = 16)
}
# adding legend at the bottom
par(mar=c(0,2,0,2)+0.1)
plot(1, type = "n", axes = F, frame.plot = F)
legend("topleft",
c("a", "b", "c", "d", "e", "f", "g", "h", "i", "j"),
fill = seq(1:10), border = "black", ncol=10,
bg = "black", bty = "n",
y.intersp = 0.8, cex = 1.2)
dev.off()
All this generates an image that looks like this:
Which is pretty much what I wanted so that's great, but I can't figure out how to add the scale (common for all 4 raster layers) in the space number 6 in this layout.
Any ideas will be greatly appreciated and put to use immediately!