How can I add some text to a ggplot2 graph in R outside the plot area?

I have a simple dataset in R.

horz <- rnorm(100)
letters <- letters[1:10]
dat <- data.frame(horz, letters)

I am plotting a graph using ggplot2.

library(ggplot2)
ggplot(dat, aes(x = horz, y = letters)) +
        geom_point(alpha = 0.3) +
        stat_summary(fun = mean, geom = "point", size = 3, color = "red")

I want to add some text on the y-axis that spans 3 categories of the y-axis variable and runs over 3 lines. Just like in the figure attached. How can I achieve that? I appreciate your help.

As documented in ggplot2 - annotate outside of plot or Add text outside plot area you can use annotate or annotation_custom to add a label outside of the plot area. Personally I prefer annotation_custom as it allows to place the label using units as well as data coordinates. Additionally, for your use case I would use gridtext::textbox_grob which allows your to draw a box around the label spanning the desired number of categories.

library(ggplot2)
library(gridtext)

set.seed(123)

horz <- rnorm(100)
letters <- letters[1:10]
dat <- data.frame(horz, letters)

ggplot(dat, aes(x = horz, y = letters)) +
  geom_point(alpha = 0.3) +
  stat_summary(fun = mean, geom = "point", size = 3, color = "red") +
  annotation_custom(
    textbox_grob(
      text = "Age 0-6m\nReference\ngroup",
      orientation = "left-rotated",
      x = unit(0, "npc") - unit(16, "pt"),
      vjust = 0,
      halign = .5,
      gp = grid::gpar(
        lineheight = 1,
        fontsize = 10
      ),
      box_gp = grid::gpar(
        col = "black"
      ),
      padding = unit(rep(5, 4), "pt")
    ),
    ymin = "b", ymax = "d"
  ) +
  # Set clip="off"
  coord_cartesian(clip = "off") +
  # Add space for the label
  theme(plot.margin = margin_part(l = 40))