library(ggplot2)

y = c(10,15,20)

x = c("group1", "group2", "group3")

ggplot() +
  geom_bar(aes(x = x,y = y), stat = "identity") +
  geom_segment(aes(x = 0.5, xend = 1.5, y = 11, yend = 11))+
  geom_segment(aes(x = 1.5, xend = 2.5, y = 16, yend = 16)) +
  geom_segment(aes(x = 2.5, xend = 3.5, y = 21, yend = 21))