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 be grateful for assistance.

Currently I have a ggplot legend that looks like this:

There are several things that I do not like about this legend that I would like to change (and that resulted in me thinking it would be easier to resort to base R graphics to do so).

In particular, I wish to eliminate the white space between the boxes in the legend and I wish to add tick marks between the boxes too. I also wish to put "5" beneath the first tick mark separating the first and second boxes; "10" beneath the tick mark separating the second and third boxes; and "20" beneath the tick mark separating the third and forth boxes. I also wish to make the boxes in the legend the same size as one of the "bins" in my plot (I use the stat_bin2d layer).

Relevant code:

ggplot()+
stat_bin2d(restaurants.df,binwidth=c(1500,2500), alpha=0.6,aes(x=X,y=Y,fill=cut(..count.., c(0,5,10,20,Inf))))+
scale_fill_manual("No. of Restaurants",labels=c("<5","5-10","10-20",">20"),values=cols, guide = guide_legend(direction = "horizontal", title.position = "top",                                      ticks=TRUE,label.position="bottom")) + 
theme(legend.background = element_rect(colour = "black"),
    legend.key = element_rect(color='black'))

Answer 1

@baptiste's comment got me interested in trying to create a plot that would become the legend. Here's my attempt. I use geom_tile to create a plot that will become the legend. The OP didn't provide sample data so I've created a plot using the built-in mtcars data, just to have something to put the legend next to. Then I use grid.arrange to create the final plot-plus-legend layout.

library(ggplot2)
library(grid)
library(gridExtra) 

## Create legend

# Fake data
dat = data.frame(x=1:4, y="Group", col=factor(1:4))

# Create a plot that will become the legend
legend = ggplot(dat, aes(x,y, fill=col)) + 
  geom_tile(show.legend=FALSE) +
  scale_x_continuous(breaks=c(1.5,2.5,3.5), labels=c(5,10,20)) +
  scale_fill_manual(values=c("yellow","orange","red","darkred")) +
  labs(y="", x="") +
  ggtitle("No. of Restaurants") +
  theme_bw() +
  theme(panel.border=element_blank(),
        axis.ticks.y=element_blank(),
        axis.text.y=element_blank())

## Create a plot to put next to the legend
p1 = ggplot(mtcars, aes(mpg, wt)) + 
  geom_point() +
  theme(plot.margin=unit(c(0,0,0,0)))

# Arrange plot and legend
grid.arrange(p1, arrangeGrob(rectGrob(gp=gpar(col=NA)), 
                             legend,
                             rectGrob(gp=gpar(col=NA)),
                             heights=c(0.42,0.16,0.42)), 
             widths=c(0.8,0.2))

Answer 2

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)