Overlay base R graphics on top of ggplot2

Question

I have a plot in ggplot and I wish to overlay a map legend that I have created with base R code. I do not know how to overlay base R graphics on top of a ggplot and would b

Answer 1

It might turn out to be easier to define your own custom legend,

library(gridExtra)
library(grid)

stripGrob <- function(cols = c("yellow","orange","red","darkred"), 
                      labels= c(5,10,20), 
                      gp=gpar(fontsize=10,fontface="italic"), vp=NULL){
  n <- length(cols)
  rg <- rasterGrob(t(cols), y=1, vjust=1, interpolate = FALSE)
  sg <- segmentsGrob(x0=seq(1/n, 1-1/n, length.out=n-1),
                     x1=seq(1/n, 1-1/n, length.out=n-1),
                     y0=unit(1,"npc") - grobHeight(rg),
                     y1=unit(1,"npc") - grobHeight(rg) - unit(2,"mm"),
                     gp=gpar(lwd=2))
  tg <- textGrob(labels, x=seq(1/n, 1-1/n, length.out=n-1),
                 unit(1,"npc") - grobHeight(rg) - grobHeight(sg) - unit(1,"mm"), 
                 vjust=1)

  stripGrob <- gTree(children = gList(rg, tg, sg), gp=gp, vp=vp)
}

qplot(1,1) +
  annotation_custom(grob=stripGrob(), xmax=1.0)