Annotating text on individual facet in ggplot2

Question

I want to annotate some text on last facet of the plot with the following code:

library(ggplot2)
p <- ggplot(mtcars, aes(mpg, wt)) + geom_point()
p <- p + facet_grid(. ~ cyl)
p <- p + annotate("text", label = "Test", size = 4, x = 15, y = 5)
print(p)

But this code annotates the text on every facet. How can I get the annotated text on only one facet?

Answer 1

Function annotate() adds the same label to all panels in a plot with facets. If the intention is to add different annotations to each panel, or annotations to only some panels, a geometry has to be used instead of annotate(). To use a geometry, such as geom_text() we need to assemble a data frame containing the text of the labels in one column and columns for the variables to be mapped to other aesthetics, as well as the variable(s) used for faceting.

Typically you'd do something like this:

ann_text <- data.frame(mpg = 15,wt = 5,lab = "Text",
                       cyl = factor(8,levels = c("4","6","8")))
p + geom_text(data = ann_text,label = "Text")

It should work without specifying the factor variable completely, but will probably throw some warnings:

Answer 2

Function annotate() adds the same label to all panels in a plot with facets. If the intention is to add different annotations to each panel, or annotations to only some panels, a geometry has to be used instead of annotate(). To use a geometry, such as geom_text() we need to assemble a data frame containing the text of the labels in one column and columns for the variables to be mapped to other aesthetics, as well as the variable(s) used for faceting. This answer exemplifies this for both facet_wrap() and facet_grid().

Here's the plot without text annotations:

library(ggplot2)

p <- ggplot(mtcars, aes(mpg, wt)) +
  geom_point() +
  facet_grid(. ~ cyl) +
  theme(panel.spacing = unit(1, "lines"))
p

Let's create an additional data frame to hold the text annotations:

dat_text <- data.frame(
  label = c("4 cylinders", "6 cylinders", "8 cylinders"),
  cyl   = c(4, 6, 8)
)
p + geom_text(
  data    = dat_text,
  mapping = aes(x = -Inf, y = -Inf, label = label),
  hjust   = -0.1,
  vjust   = -1
)

Alternatively, we can manually specify the position of each label:

dat_text <- data.frame(
  label = c("4 cylinders", "6 cylinders", "8 cylinders"),
  cyl   = c(4, 6, 8),
  x     = c(20, 27.5, 25),
  y     = c(4, 4, 4.5)
)

p + geom_text(
  data    = dat_text,
  mapping = aes(x = x, y = y, label = label)
)

We can also label plots across two facets:

dat_text <- data.frame(
  cyl   = c(4, 6, 8, 4, 6, 8),
  am    = c(0, 0, 0, 1, 1, 1)
)
dat_text$label <- sprintf(
  "%s, %s cylinders",
  ifelse(dat_text$am == 0, "automatic", "manual"),
  dat_text$cyl
)
p +
  facet_grid(am ~ cyl) +
  geom_text(
    size    = 5,
    data    = dat_text,
    mapping = aes(x = Inf, y = Inf, label = label),
    hjust   = 1.05,
    vjust   = 1.5
  )

Notes:

• You can use -Inf and Inf to position text at the edges of a panel.
• You can use hjust and vjust to adjust the text justification.
• The text label data frame dat_text should have a column that works with your facet_grid() or facet_wrap().

Answer 3

If anyone is looking for an easy way to label facets for reports or publications, the egg (CRAN) package has pretty nifty tag_facet() & tag_facet_outside() functions.

library(ggplot2)

p <- ggplot(mtcars, aes(qsec, mpg)) + 
  geom_point() + 
  facet_grid(. ~ am) +
  theme_bw(base_size = 12)

# install.packages('egg', dependencies = TRUE)
library(egg)

Tag inside

Default

tag_facet(p)

Note: if you want to keep the strip text and background, try adding strip.text and strip.background back in theme or remove theme(strip.text = element_blank(), strip.background = element_blank()) from the original tag_facet() function.

tag_facet <- function(p, open = "(", close = ")", tag_pool = letters, x = -Inf, y = Inf, 
                      hjust = -0.5, vjust = 1.5, fontface = 2, family = "", ...) {

  gb <- ggplot_build(p)
  lay <- gb$layout$layout
  tags <- cbind(lay, label = paste0(open, tag_pool[lay$PANEL], close), x = x, y = y)
  p + geom_text(data = tags, aes_string(x = "x", y = "y", label = "label"), ..., hjust = hjust, 
                vjust = vjust, fontface = fontface, family = family, inherit.aes = FALSE) 
}

Align top right & use Roman numerals

tag_facet(p, x = Inf, y = Inf, 
          hjust = 1.5,
          tag_pool = as.roman(1:nlevels(factor(mtcars$am))))

Align bottom left & use capital letters

tag_facet(p, 
          x = -Inf, y = -Inf, 
          vjust = -1,
          open = "", close = ")",
          tag_pool = LETTERS)

Define your own tags

my_tag <- c("i) 4 cylinders", "ii) 6 cyls")
tag_facet(p, 
          x = -Inf, y = -Inf, 
          vjust = -1, hjust = -0.25,
          open = "", close = "",
          fontface = 4,
          size = 5,
          family = "serif",
          tag_pool = my_tag)

Tag outside

p2 <- ggplot(mtcars, aes(qsec, mpg)) + 
  geom_point() + 
  facet_grid(cyl ~ am, switch = 'y') +
  theme_bw(base_size = 12) +
  theme(strip.placement = 'outside')

tag_facet_outside(p2)

Edit: adding another alternative using the stickylabeller package

- `.n` numbers the facets numerically: `"1"`, `"2"`, `"3"`...
- `.l` numbers the facets using lowercase letters: `"a"`, `"b"`, `"c"`...
- `.L` numbers the facets using uppercase letters: `"A"`, `"B"`, `"C"`...
- `.r` numbers the facets using lowercase Roman numerals: `"i"`, `"ii"`, `"iii"`...
- `.R` numbers the facets using uppercase Roman numerals: `"I"`, `"II"`, `"III"`...

# devtools::install_github("rensa/stickylabeller")
library(stickylabeller)

ggplot(mtcars, aes(qsec, mpg)) + 
  geom_point() + 
  facet_wrap(. ~ am, 
             labeller = label_glue('({.l}) am = {am}')) +
  theme_bw(base_size = 12)

^{Created by the reprex package (v0.2.1)}

Answer 4

I think for the answer above lab="Text" is useless, the code below is also ok.

ann_text <- data.frame(mpg = 15,wt = 5,
                       cyl = factor(8,levels = c("4","6","8")))
p + geom_text(data = ann_text,label = "Text" )

However if you want to label differently in different sub-graphs, it will be ok in this way:

ann_text <- data.frame(mpg = c(14,15),wt = c(4,5),lab=c("text1","text2"),
                       cyl = factor(c(6,8),levels = c("4","6","8")))
p + geom_text(data = ann_text,aes(label =lab) )

Answer 5

Expanding slightly on joran's excellent answer, to clarify how the label dataframe works.

You can think of "mpg" and "wt" as the x and y coordinates, respectively (I find it easier to keep track of the original variable names than renaming them, as in Kamil's also-excellent answer). You need one row per label, and the "cyl" column shows which facet each row is associated with.

ann_text<-data.frame(mpg=c(25,15),wt=c(3,5),cyl=c(6,8),label=c("Label 1","Label 2"))

ann_text
>  mpg wt cyl  label
>  25  3   6   Label 1
>  15  5   8   Label 2

p <- ggplot(mtcars, aes(mpg, wt)) + geom_point()
p <- p + facet_grid(. ~ factor(cyl))
p + geom_text(data = ann_text,label=ann_text$label)

Answer 6

I did not know about the egg package, so here is a plain ggplot2 package solution

library(tidyverse)
library(magrittr)
Data1=data.frame(A=runif(20, min = 0, max = 100), B=runif(20, min = 0, max = 250), C=runif(20, min = 0, max = 300))
Data2=data.frame(A=runif(20, min = -10, max = 50), B=runif(20, min = -5, max = 150), C=runif(20, min = 5, max = 200))

bind_cols(
Data1 %>% gather("Vars","Data_1"),
Data2 %>% gather("Vars","Data_2")
) %>% select(-Vars1) -> Data_combined

Data_combined %>%
  group_by(Vars) %>%
  summarise(r=cor(Data_1,Data_2),
            r2=r^2,
            p=(pt(abs(r),nrow(.)-2)-pt(-abs(r),nrow(.)-2))) %>%
  mutate(rlabel=paste("r:",format(r,digits=3)),
         plabel=paste("p:",format(p,digits=3))) ->
  label_df 

label_df %<>% mutate(x=60,y=190)

Data_combined %>%
  ggplot(aes(x=Data_1,y=Data_2,color=Vars)) +
  geom_point() + 
  geom_smooth(method="lm",se=FALSE) +
  geom_text(data=label_df,aes(x=x,y=y,label=rlabel),inherit.aes = FALSE) + 
  geom_text(data=label_df,aes(x=x,y=y-10,label=plabel),inherit.aes = FALSE) + 
    facet_wrap(~ Vars)