Plot a second colorkey using levelplot and grid graphics in R

levelplot_second_colorkey.R

library(raster)
library(rasterVis)
library(RColorBrewer)
library(grid)
library(viridisLite)

r1 <- raster(matrix(runif(100), 10))
r2 <- raster(matrix(runif(100), 10))
xy <- xyFromCell(r1, sample(ncell(r1), 10), spatial=TRUE)
dif <- raster::extract(r2, xy) - raster::extract(r1, xy)

p <- levelplot(r, margin=FALSE, colorkey=list(height=0.6), 
               at=seq(0, 1, length=100), xlab='', ylab='', 
               col.regions=viridisLite::viridis,
               main='A plot', 
               par.settings=list(layout.heights=list(axis.bottom=1.6)))

ramp <- colorRampPalette(brewer.pal(11, 'RdBu'))

k <- draw.colorkey(
  list(space='bottom', at=seq(-1, 1, length=100), height=0.25,  
  col=ramp)
)
k$framevp$y <- unit(0.05, "npc")
k$framevp$name <- 'key2'

p + layer(
  sp.points(xy, pch=21, cex=1.5, lwd=2,
            col=ramp(100)[as.numeric(cut(dif, seq(-1, 1, length=100)))])
)
grid.draw(k)
seekViewport('key2')
pushViewport(grid::current.vpTree(all=FALSE)$children[[1]])
grid.text('difference', 0.5, 1.2, hjust=0.5, vjust=0, gp=gpar(font=2))