ggplot2 - annotate outside of plot

Question

I would like to associate sample size values with points on a plot. I can use geom_text to position the numbers near the points, but this is messy. It would be much cleaner to line them up along the outside edge of the plot.

For instance, I have:

df=data.frame(y=c("cat1","cat2","cat3"),x=c(12,10,14),n=c(5,15,20))

ggplot(df,aes(x=x,y=y,label=n))+geom_point()+geom_text(size=8,hjust=-0.5)

Which produces this plot:

I would prefer something more like this:

I know I can create a second plot and use grid.arrange (a la this post) but it would be tedious to determine the spacing of the textGrobs to line up with the y-axis. Is there an easier way to do this? Thanks!

Answer 1

This is now straightforward with ggplot2 3.0.0, since now clipping can be disabled in plots by using the clip = 'off' argument in coordinate functions such as coord_cartesian(clip = 'off') or coord_fixed(clip = 'off'). Here's an example below.

    # Generate data
    df <- data.frame(y=c("cat1","cat2","cat3"),
                     x=c(12,10,14),
                     n=c(5,15,20))
    
    # Create the plot
    ggplot(df,aes(x=x,y=y,label=n)) +
      geom_point()+
      geom_text(x = I(1), # Set text's position to the right end of the plot
                hjust = 0,
                size = 8) +
      coord_cartesian(xlim = c(10, 14), # This focuses the x-axis on the range of interest
                      clip = 'off') +   # This keeps the labels from disappearing
      theme(plot.margin = unit(c(1,3,1,1), "lines")) # This widens the right margin

Thanks to @captain-hat for the suggestion to use I(1) in geom_text(x = I(1) to set the text position to the far right end of the plot.

Answer 2

Another option which is similar in spirit to the approach by @jan-glx but using just vanilla ggplot2 would be to use the so-called secondary axis trick which means to use a secondary or duplicated axis to add the annotations.

However, in the case of a discrete scale this is slightly more involved as a discrete scale does not allow for a secondary axis. Hence, we have to switch to a continuous scale first by converting the discrete y axis variable to a numeric using e.g. as.numeric(factor(...)).

library(ggplot2)

df$y_num <- as.numeric(factor(df$y))

ggplot(df, aes(x = x, y = y_num, label = n)) +
  geom_point() +
  scale_y_continuous(
    # Fix the breaks
    breaks = unique(df$y_num),
    labels = df$y,
    # Set default discrete scale amount of expansion
    expand = c(0, .6),
    sec.axis = dup_axis(
      breaks = unique(df$y_num),
      labels = paste0("n = ", df$n)
    )
  ) +
  theme(
    # Multiply by `.pt` to convert to `geom_text` font size
    # (the latter is measured in "mm", while the axis text uses "pt")
    axis.text.y.right = element_text(size = 8 * .pt),
    axis.ticks.y.right = element_blank(),
    axis.title.y.right = element_blank()
  )

Answer 3

This particular example might be a case for ggh4x::guide_axis_manual

# remotes::install_github("teunbrand/ggh4x")
library(ggplot2)

df <- data.frame(y=c("cat1","cat2","cat3"), x=c(12,10,14), n=c(5,15,20))

ggplot(df, aes(x=x, y=y)) +
  geom_point() +
  guides(y.sec=ggh4x::guide_axis_manual(title = element_blank(), breaks = df$y, labels = paste0("n=",df$n)))

^{Created on 2023-08-24 with reprex v2.0.2}

Answer 4

Another option could be using annotate from ggplot2 which is almost the same as using geom_text:

library(ggplot2)
df=data.frame(y=c("cat1","cat2","cat3"),x=c(12,10,14),n=c(5,15,20))
ggplot(df,aes(x=x,y=y)) + 
  geom_point() + 
  annotate("text", x = max(df$x) + 0.5, y = df$y, label = df$n, size = 8) +
  coord_cartesian(xlim = c(min(df$x), max(df$x)), clip = "off") +
  theme(plot.margin = unit(c(1,3,1,1), "lines"))

^{Created on 2022-08-14 by the reprex package (v2.0.1)}

Answer 5

Simplier solution based on grid

require(grid)

df = data.frame(y = c("cat1", "cat2", "cat3"), x = c(12, 10, 14), n = c(5, 15, 20))

p <- ggplot(df, aes(x, y)) + geom_point() + # Base plot
theme(plot.margin = unit(c(1, 3, 1, 1), "lines"))

p

grid.text("20", x = unit(0.91, "npc"), y = unit(0.80, "npc"))
grid.text("15", x = unit(0.91, "npc"), y = unit(0.56, "npc"))
grid.text("5", x = unit(0.91, "npc"), y = unit(0.31, "npc"))

Answer 6

You don't need to be drawing a second plot. You can use annotation_custom to position grobs anywhere inside or outside the plotting area. The positioning of the grobs is in terms of the data coordinates. Assuming that "5", "10", "15" align with "cat1", "cat2", "cat3", the vertical positioning of the textGrobs is taken care of - the y-coordinates of your three textGrobs are given by the y-coordinates of the three data points. By default, ggplot2 clips grobs to the plotting area but the clipping can be overridden. The relevant margin needs to be widened to make room for the grob. The following (using ggplot2 0.9.2) gives a plot similar to your second plot:

library (ggplot2)
library(grid)

df=data.frame(y=c("cat1","cat2","cat3"),x=c(12,10,14),n=c(5,15,20))

p <- ggplot(df, aes(x,y)) + geom_point() +            # Base plot
     theme(plot.margin = unit(c(1,3,1,1), "lines"))   # Make room for the grob

for (i in 1:length(df$n))  {
p <- p + annotation_custom(
      grob = textGrob(label = df$n[i], hjust = 0, gp = gpar(cex = 1.5)),
      ymin = df$y[i],      # Vertical position of the textGrob
      ymax = df$y[i],
      xmin = 14.3,         # Note: The grobs are positioned outside the plot area
      xmax = 14.3)
 }    

# Code to override clipping
gt <- ggplot_gtable(ggplot_build(p))
gt$layout$clip[gt$layout$name == "panel"] <- "off"
grid.draw(gt)