eccentric circles legend

library(ggforce)
library(dplyr)

l <- data.frame(a = c(100, 50, 10)) %>% 
  mutate(
    r = sqrt(a/pi),
    x0 = 0,
    y0 = r
    )

ggplot(l) +
  geom_circle(aes(x0 = x0, y0 = y0, r = r)) +
  geom_text(aes(x = 8, y = y0, label = a), hjust = 0) +
  geom_segment(aes(x = x0 + r, y = y0, xend = 7.75, yend = y0), linetype = "dashed") +
  coord_fixed() +
  theme_void()