How to make a superscript expression in ggplot2 legend

Question

for some reason the expression I am trying to use for my legend isn't creating the superscript (using ggplot2). Could someone please direct me where I'm going wrong? I need to have the two lines as I want to maximize my plot area.

Reproducible example (although not in ggplot2):

plot(1:1, main=expression(paste('Mean driving\nspeed (km h^-1)')))

Thank you so much

Answer 1

The equivalent ggplot with the superscript using both expression() and ggtext (in the title, subtitle and legend):

library(tidyverse)
library(ggtext)

df <- tibble(x = 1, y = 1)

df |>
  ggplot(aes(x, y, colour = x)) +
  geom_point() +
  labs(
    title = "Mean driving<br>speed (km h<sup>-1</sup>)",
    # subtitle = expression(paste("km", h^-1)), # using expression
    colour = "Mean driving<br>speed (km h<sup>-1</sup>)"
  ) +
  theme(
    plot.title = element_markdown(),
    legend.title = element_markdown()
  )

^{Created on 2024-04-12 with reprex v2.1.0}