Add a fake legend annotation to a ggplot

fakelegend.R

library(ggplot2)
library(dplyr)

## fake data
d <- mtcars %>%
  group_by(cyl) %>%
  summarise(lower = min(disp),
            med = median(disp),
            upper = max(disp))
p <- ggplot(d, aes(x = factor(cyl), y = med)) +
  geom_pointrange(aes(ymin = lower, ymax = upper)) +
  geom_point(aes(factor(cyl), lower)) +
  geom_point(aes(factor(cyl), upper))

## configure legend
config <- tibble::tribble(
  ~ x, ~ y, ~ s, ~ t,
  1, 1, 1, "Min",
  3, 1, 2, "Median",
  6, 1, 1, "Max"
)
lcol <- "forestgreen"
adj <- 0.1

## build fake legend
legend <- ggplot(config, aes(x, y)) +
  geom_point(aes(size = s), show.legend = FALSE, col = lcol) +
  scale_size_continuous(range = c(2, 4)) +
  coord_cartesian(ylim = c(1 - adj, 1 + adj)) +
  geom_text(aes(y = y - 0.5 * adj, label = t), size = 6, col = lcol) +
  geom_line(lwd = 2, col = lcol) +
  theme_void()
legend

## combine
p +
  theme(plot.margin = unit(c(5.5, 5.5, 100, 5.5), "points")) +
  annotation_custom(
    ggplotGrob(legend),
    xmin = 1,
    xmax = 3,
    ymin = -50,
    ymax = 40
  )