How to add a custom legend to plot with ggplot?

Question

I want to add a new legend to my plot. But I hope the legend is all customized. I add the label for every point by geom_text_repel. The new legend describes the of character of the labels.

Answer 1

You can create a legend by creating "dummy" data that contains the legend key labels. You would then "plot" the dummy data in order to generate the legend, but use blank symbols so that nothing actually gets plotted.

library(ggplot2)
theme_set(theme_classic())    

# Fake data for plotting
set.seed(2)
val = sapply(sample(1:4,30,replace=TRUE), function(x) paste(sort(sample(c('c','u','x','t'), x)), collapse=""))
dat = data.frame(x=runif(30), y=runif(30), val) 

# Dummy data for creating the legend
leg = data.frame(x1=rep(0,4), y1=rep(0,4), ll = c("c: coor","u: url","x: xss","t: text"))

ggplot(data=dat, aes(x,y)) + 
  geom_text(aes(label=val)) +
  geom_point(data=leg, aes(x1, y1, colour=ll)) +
  theme(legend.key.size=unit(15,"pt"),
        legend.title=element_blank(),
        legend.margin=margin(l=0),
        legend.text=element_text(size=12)) +
  scale_colour_manual(values=rep("#00000000", 4))

You could also use geom_text to place the "legend" annotations directly:

leg = data.frame(ll = sort(c("c: coor","u: url","x: xss","t: text")))
leg$y = seq(mean(dat$y) + 0.05*diff(range(dat$y)), 
            mean(dat$y) - 0.05*diff(range(dat$y)),
            length=4)
leg$x = 1.07 * max(dat$x)

ggplot(data=dat, aes(x,y)) + 
  geom_text(aes(label=val)) +
  geom_text(dat=leg, aes(label=ll), hjust=0, colour="red") +
  annotate(xmin=1.05 * max(dat$x), xmax=1.18 * max(dat$x), ymin=0.95*min(leg$y), ymax=1.04*max(leg$y), 
           geom="rect", fill=NA, colour="black") + 
  scale_x_continuous(limits=c(min(dat$x), 1.18*max(dat$x)))