How to add “grouped” labels using ggplot in R?

Question

I have a made a heatmap in R using ggplot.

Example

# Libraries
library(tidyverse)

# Create data frame
df <- data.frame(test = rep(c("testA-01", "testA-02", "testA-03", "testB-01", "testB-02", "testB-03", "testC-01", "testC-02", "testC-03"),3), 
                 time = c( rep(0,9), rep(1, 9), rep(2, 9) ), 
                 score = sample(1:10, 27, replace = TRUE) )

# Create heatmap
ggplot(data = df, mapping = aes(x = time, y = test)) +
  geom_tile(mapping = aes(fill = score, width=0.9, height=0.9)) +
  scale_fill_gradientn(limits = c(1,10), colours=c("grey95", "grey40", "red"), na.value = "white" ) +
  scale_y_discrete(name = "Test", limits = c("testC-03", "testC-02", "testC-01", "testB-03", "testB-02", "testB-01", "testA-03",
                                                "testA-02", "testA-01")) +
  theme_classic()

This lead to the following plot:

I would like to bundle the labels on the y-axis, so that I do not repeat "Test[letter]" three times for each test. I am able to do it manually, however, I thought maybe it is possible to use ggplot. The first part of the solution is to remove the "Test[letter]" part from the limits of scale_y_discrete() . Next I would like to add the labels vertically and grouped per test on the y-axis (preferably with a vertical line grouping the tests), like this:

Expected Result

Is this possible in ggplot? And if so, how do you do this?

Answer 1

Some rearrangements of the dataframe eases the plotting:

df <- data.frame(batch = c( rep("TestA",9), rep("TestB", 9), rep("TestC", 9) ), 
                 test = rep(c(1,2,3),9), 
                 time = rep(c(0,0,0,1,1,1,2,2,2),3), 
                 score = sample(1:10, 27, replace = TRUE) )

Plot

With the facet_grid() function it is possible to facet the data in the plot. Using the annotation_custom() function of ggplot and coord_cartesian() , I was able to add 'grouping' lines.

library(tidyverse)
library(grid)

ggplot(data = df, mapping = aes(x = time, y = test)) +
geom_tile(mapping = aes(fill = score, width=0.9, height=0.9)) +
scale_fill_gradientn(limits = c(1,10), colours=c("grey95", "grey40", "red"), na.value = "white" ) +
theme_classic() +
scale_y_reverse() +
facet_grid(batch ~ ., space="free_x", scales="free_y", switch="y") +
theme(strip.placement = "outside",
      strip.background = element_rect(fill=NA,colour=NA),
      panel.spacing=unit(0,"cm"), axis.title.y = element_blank()) +
annotation_custom(grob = linesGrob(), xmin = -0.75, xmax = -0.75, ymin = -3.25, ymax = -0.75) +
coord_cartesian(clip="off")