Add data-specific text to each plot within facet grid in R

Question

I have a histogram which I want to facet based on three separate factors. I then want to add two lines of text in the top right-hand corner of each facet. The text is data dependent, and will be different for every subset of the data.

Here I use the Heart Attack Analysis data on Kaggle. I download and unzip the data, then read-in the heart.csv. I separate out the data into 3 factors (sex, slp, exng) and get the maximum and minimum ages within each subset. Then I plot the ages by factor in ggplot. I want the maximum and minimum ages in the top right-hand corner of the plot. But I could only figure out how to do this in a single plot (without the facet grid step).

Here's the code:

# Load data
heart <- read.csv(file = 'C:/FilePath/heart.csv')


# Split data into subsets based on our three factors
hrt_grps <- heart %>%
  group_split(sex,slp,exng)

# Get the max and min within each subset (and some other stats as well)
hrt_grps_smry <- tibble::tibble()
colnames(hrt_grps_smry) <- c("sex","slp","exng", "max_d", "min_d", 
                              "mean_d","t.p-val","t.conf.L","t.conf.U")


# Iterate through every element in the hrt_grps group-split and populate the rows of hrt_grps_smry df
for(i in 1:length(hrt_grps)){
  t.tst <- t.test(x = hrt_grps[[i]]$age,
                  alternative = "two.sided")
  
  hrt_grps_smry[i,"sex"]       <- hrt_grps[[i]]$sex[1]
  hrt_grps_smry[i,"slp"]       <- hrt_grps[[i]]$slp[1]
  hrt_grps_smry[i,"exng"]      <- hrt_grps[[i]]$exng[1]
  hrt_grps_smry[i,"max_d"]     <- max(hrt_grps[[i]]$age)
  hrt_grps_smry[i,"min_d"]     <- min(hrt_grps[[i]]$age)
  hrt_grps_smry[i,"mean_d"]    <- mean(hrt_grps[[i]]$age)
  hrt_grps_smry[i,"t.p-val"]   <- t.tst$p.value
  hrt_grps_smry[i,"t.conf.L"]  <- t.tst$conf.int[[1]]
  hrt_grps_smry[i,"t.conf.U"]  <- t.tst$conf.int[[2]]

}

# Plot single histogram with max and min in top right-hand corner (successful):
heart %>%
  # This line is because in my real data it is very important that I control the order of the facets
  mutate(across(slp,factor, levels = c(2,0,1))) %>%
  ggplot(aes(x=age)) +
  geom_histogram(bins = 35) +
 # facet_grid(sex ~ slp ~ exng) +
  geom_text(
    data = hrt_grps_smry, 
    aes(x=5, y = median(density(heart$age)$y)), 
    label = max(hrt_grps_smry$max_d), vjust = -35, hjust = -40, 
    size = 4, angle = 0, colour = "gray10") +
  geom_text(
    data = hrt_grps_smry, 
    aes(x=5, y = median(density(heart$age)$y)), 
    label = min(hrt_grps_smry$min_d), vjust = -32, hjust = -40, 
    size = 4, angle = 0, colour = "gray10") +
  ylab("Count")

# Plot facet-grid histogram of ages with the max and min in the top right-hand corner
heart %>%
  mutate(across(slp,factor, levels = c(2,0,1))) %>%
  ggplot(aes(x=age)) +
  geom_histogram(bins = 35) +
  facet_grid(sex ~ slp ~ exng) +
  geom_text(
    data = hrt_grps_smry, 
    aes(x=5, y = median(density(heart$age)$y)), 
    label = hrt_grps_smry$max_d[1], vjust = -4.1, hjust = -18, 
    size = 4, angle = 0, colour = "gray10") +
  geom_text(
    data = hrt_grps_smry, 
    aes(x=5, y = median(density(heart$age)$y)), 
    label = hrt_grps_smry$min_d[1], vjust = -2.8, hjust = -18, 
    size = 4, angle = 0, colour = "gray10") +
  ylab("Count")

I've only figured out how to grab the max and min values for the first subset. I have not figured out how to iterate through subsets and keep all plots in the same facet grid ggplot object.

Shafee · Accepted Answer · 2022-08-14 06:46:04Z

The simplest way of doing this, as far as I know, is to add those min_d and max_d variables in the original dataset and then use them in geom_text

library(tidyverse)

# Load data
heart <- read.csv(file = 'test/heart.csv')

# calculate group wise max and min age and also create a group id variable 
# which will be used later to do merging
min_max_df <- heart %>% 
  group_by(sex, slp, exng) %>% 
  summarise(
    id = cur_group_id(),
    max_d = max(age),
    min_d = min(age),
    .groups = "drop"
  )

# merge the group wise min and max age with the main data by group id 
heart <- heart %>% 
  group_by(sex, slp, exng) %>% 
  mutate(
    id = cur_group_id()
  ) %>% 
  ungroup() %>% 
  left_join(
    min_max_df %>% select(id, max_d, min_d),
    by = "id"
  )


heart %>%
  mutate(across(slp,factor, levels = c(2,0,1))) %>% 
ggplot(aes(x=age)) +
  geom_histogram(bins = 35) +
  geom_text(aes(x = 72, y = 8, label = paste0("Max age: ", max_d)),
            size = 3, color = colorspace::lighten("black", amount = 0.5)) +
  geom_text(aes(x = 72, y = 6, label = paste0("Min age: ", min_d)),
            size = 3, color = colorspace::lighten("black", amount = 0.5)) +
  facet_grid(sex ~ slp ~ exng)

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