GGPLOT: How to Display the Last Value of Each Line as Label

Prerequisites

Required R packages:

• tidyverse: easy data manipulation and visualization
• ggrepel: provides geoms for ggplot2 to repel overlapping text labels: geom_text_repel() and geom_label_repel()

# Load required R packages
library(tidyverse)
library(ggrepel)
# Set ggplot2 default theme to theme_bw()
theme_set(theme_bw())

# Demo data
df <- tibble::tribble(
      ~Species, ~Petal.Length, ~Petal.Width, ~Sepal.Length, ~Sepal.Width,
      "setosa",         1.462,        0.246,         5.006,        3.428,
  "versicolor",          4.26,        1.326,         5.936,         2.77,
   "virginica",         5.552,        2.026,         6.588,        2.974
  )
df

## # A tibble: 3 x 5
##   Species    Petal.Length Petal.Width Sepal.Length Sepal.Width
##   <chr>             <dbl>       <dbl>        <dbl>       <dbl>
## 1 setosa             1.46       0.246         5.01        3.43
## 2 versicolor         4.26       1.33          5.94        2.77
## 3 virginica          5.55       2.03          6.59        2.97

# Transform the data into long format
# Put Petal.Length to Sepal.Width in the same column
df_long <- df %>%
  pivot_longer(
    Petal.Length:Sepal.Width,
    names_to = "variable", values_to = "value"
  )
df_long

## # A tibble: 12 x 3
##   Species    variable     value
##   <chr>      <chr>        <dbl>
## 1 setosa     Petal.Length 1.46 
## 2 setosa     Petal.Width  0.246
## 3 setosa     Sepal.Length 5.01 
## 4 setosa     Sepal.Width  3.43 
## 5 versicolor Petal.Length 4.26 
## 6 versicolor Petal.Width  1.33 
## # … with 6 more rows

Using ggrepel to add labels to the line ends

# Basic line plot
lp <- ggplot(df_long, aes(x = Species, y = value, group = variable)) +
  geom_line(aes(color = variable)) +
  geom_point() +
  theme(legend.position = "top")

# Filter the last values and add onto the line plot
# Corresponds to the `virginica` species
data_ends <- df_long %>% filter(Species == "virginica")
lp + 
  geom_text_repel(
    aes(label = value), data = data_ends,
    fontface ="plain", color = "black", size = 3
    )

# Use variable names as labels
lp2 <- ggplot(df_long, aes(x = Species, y = value, group = variable)) +
  geom_line() +
  geom_point() 
lp2 + 
  geom_text_repel(
    aes(label = variable), data = data_ends,
    color = "black", size = 3
    )

# Demo data
df2 <- Orange
head(df2)

## Grouped Data: circumference ~ age | Tree
##   Tree  age circumference
## 1    1  118            30
## 2    1  484            58
## 3    1  664            87
## 4    1 1004           115
## 5    1 1231           120
## 6    1 1372           142

# Filter the last values
data_ends <- df2 %>% 
  group_by(Tree) %>% 
  top_n(1, age) 
data_ends

## # A tibble: 5 x 3
## # Groups:   Tree [5]
##   Tree    age circumference
##   <ord> <dbl>         <dbl>
## 1 1      1582           145
## 2 2      1582           203
## 3 3      1582           140
## 4 4      1582           214
## 5 5      1582           177

ggplot(df2, aes(age, circumference)) +
  geom_line(aes(color = Tree)) +
  geom_text_repel(
    aes(label = circumference), data = data_ends, 
    size = 3
    )

Using a secondary y axis to show the line labels

Key R functions: The ggplot2 scale_y_continuous() function is used in combination with the argument sec.axis to create a second axis on the right. The numbers to be displayed at breaks is defined by the vector of values corresponding to the line ends.

# Pull the vector of last values
data_ends <- df2 %>% 
  group_by(Tree) %>% 
  top_n(1, age) %>%
  pull(circumference)
data_ends

## [1] 145 203 140 214 177

# Create the line plot with labels
ggplot(df2, aes(x = age, y = circumference)) +
      geom_line(aes(color = Tree)) +
      scale_y_continuous(sec.axis = sec_axis(~ ., breaks = data_ends))

Conclusion

This article describes how to display the last value of each line as a label.