Add second character axis to ggplot2

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I'm looking to present the results of a user survey of semantic differential scores as a boxplot, with the first semantic displayed along the default x-axis, and the second semantic displayed along a secondary top x-axis. In simple terms this can be represented as

df <- data.frame(sem1 = c("good", "big", "red"), sem2 = c("bad", "small", "blue"), a = c(4,6,1), b = c(6,2,6), c = c(8,3,7))`
df %>% pivot_longer(!c(sem1, sem2), names_to = "user", values_to = "score") %>%
  ggplot() +
  geom_boxplot(aes(x = sem1, y = score)) +
  ylim(1,10)

Which produces the below chart. What I would really like to do is add the characters from sem2 (i.e. "bad", "small", "blue") to a secondary axis along the top of the graph. Everything I've seen using sec_axis seems geared towards transformations of numerical axes rather than the addition of a distinct second axis. I believe I could possibly use annotation_custom to add each semantic one by one, but in my actual application with ~15 semantic pairs to display this could get quite cumbersome. Is there a more elegant way?

sample boxplot

ggplot2 r
1 Answers

A possible option:

library(tidyverse)

df <- data.frame(sem1 = c("good", "big", "red"), 
                 sem2 = c("bad", "small", "blue"), 
                 a = c(4, 6, 1), b = c(6, 2, 6), 
                 c = c(8, 3, 7)) %>%
  pivot_longer(!c(sem1, sem2), names_to = "user", values_to = "score") 

df %>%
  ggplot(aes(sem1, score)) +
  geom_boxplot() +
  geom_label(aes(y = max(score * 1.2), label = sem2)) +
  ylim(1, 10)

Created on 2022-05-19 by the reprex package (v2.0.1)

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