Displaying correlation coefficients in bold with ggpubr Stat_cor

I use stat_cor() to display the pearson correlation coefficients on a boxplot. To favor the visibility of the coefficients, I am trying to display them in bold. I am still quite new to R, and I tried to solve the issue by myself with this code :

ggplot(iris, aes(x = Sepal.Width, y = Sepal.Length)) +
  geom_point(aes(color = Species)) +
  stat_smooth(method = lm, aes(color = Species), se = FALSE) +
  stat_cor(aes(color = Species, label = after_stat(r.label)), 
  r.accuracy = 0.01, label.x = 4, label.y = c(8, 7.8, 7.6), 
  fontface = "bold") +
  scale_color_manual(values = c("grey70", "gold", "green3")) +
  theme_classic() +
  theme(
        legend.background = element_rect(fill="grey90", 
                                         size=0.5, linetype="solid"))

This method didn’t work, and I couldn’t figure out how to do it in theme(), even though I believe there is probably a way.

>Solution :

As a general rule: The theme() will have no effect on text labels. It sets the overall look of the plot and will affect only the so-called non-data ink. For text labels you have to set the font face in the geom or the stat. However, stat_cor uses ?plotmath expressions to create the labels, i.e. make the R or … italic. And in that the face of the labels can’t be set or changed via fontface=.

Hence, to make the labels bold requires to create the label yourself inside after_stat:

library(ggplot2)
library(ggpubr)

ggplot(iris, aes(x = Sepal.Width, y = Sepal.Length)) +
  geom_point(aes(color = Species)) +
  stat_smooth(method = lm, aes(color = Species), se = FALSE) +
  stat_cor(
    aes(
      color = Species,
      label = after_stat(
        paste0(
          "bolditalic(R)~",
          "bold(`=`)~",
          "bold('", r, "')"
        )
      )
    ),
    r.accuracy = 0.01,
    label.x = 4, label.y = c(8, 7.8, 7.6),
  ) +
  scale_color_manual(values = c("grey70", "gold", "green3")) +
  theme_classic() +
  theme(
    legend.background = element_rect(
      fill = "grey90",
      size = 0.5, linetype = "solid"
    )
  )
#> Warning: The `size` argument of `element_rect()` is deprecated as of ggplot2 3.4.0.
#> ℹ Please use the `linewidth` argument instead.
#> This warning is displayed once every 8 hours.
#> Call `lifecycle::last_lifecycle_warnings()` to see where this warning was
#> generated.
#> `geom_smooth()` using formula = 'y ~ x'