multi-facet_legend_ggplot2.R

library(ggplot2)
set.seed(42)
x  <- 1:20
y1 <- 2 * x + 3
y2 <- rep(y1,2) + rnorm(40,0,4)
y3 <- rep(y1,2) + rnorm(40,0,10)

y <- c(y2, y3)
type <- factor(rep(c("clyde","irving"), each = 40))
facet <- factor(rep(c("a","b"), each = 20))
df <- data.frame(x = rep(x, 4), y, colour, facet)

ggplot(df, aes(x, y, col = type, shape = type)) +
  geom_point() +
  # Specifying the same legend title  merges the legends. From:
  # https://stackoverflow.com/a/49791164
  labs(x = "x", y = "y", col = "What I want", shape = "What I want") +
  geom_abline(intercept = 3, slope = 2) +
  facet_wrap(vars(facet)) +
  scale_colour_manual(values = c("blue","red")) + 
  scale_shape_manual(values = c(20, 3))