R: Add multiple colors to the y-axis label in ggplot2 figure

AFAIK there is no out of the box option to achieve that, e.g. via ggtext we could only set the text color using some HTML/CSS and ggtext::element_markdown. Hence, the best "hack" I could come up with is to add the pills or boxes using ggtext::geom_textbox which however requires some fiddling or computations to set the x position of the text box, the box width and the box margin:

library(ggplot2)
library(ggtext)

base <- mtcars |> 
  dplyr::mutate(
    color = 
      c("BLUE", "RED") |> rep_len(dplyr::n()) |> 
      factor()
  ) |> 
  ggplot(aes(x = mpg, y = color)) +
  geom_point() +
  scale_y_discrete(labels = ~ "BLUEBLUE") + 
  scale_fill_identity() +
  labs(y = NULL) +
  coord_cartesian(clip = "off") +
  theme(axis.text.y.left = element_text(color = "transparent"))

base +
  ggtext::geom_textbox(data = ~ dplyr::distinct(.x, color), aes(fill = tolower(color)), label = "BLUE", hjust = 1, box.margin = unit(5, "pt"),
                       x = range(mtcars$mpg)[1] - .05 * diff(range(mtcars$mpg)), color = "transparent",
                       width = unit(.75, "inch"))

And another option would be to simply use geom_label which however requires some more fiddling as e.g. ggtext::geom_textbox offers some additional arguments like box.margin and width.

base +
  geom_label(data = ~ dplyr::distinct(.x, color), aes(fill = tolower(color)), label = "BLUE", hjust = 1, 
             label.r = unit(0.25, "lines"),
                       x = range(mtcars$mpg)[1] - .05 * diff(range(mtcars$mpg)) - .25, color = "transparent")

An option without using additional packages would be to use annotation_custom. In this case, we can use roundrectGrob:

library(ggplot2)

mtcars |> 
  dplyr::mutate(
    color = 
      c("BLUE", "RED") |> rep_len(dplyr::n()) |> 
      factor()
  ) |> 
  ggplot(aes(x = mpg, y = color)) +
  geom_point() +
  labs(y = "") +
  scale_y_discrete(labels = rep(paste0(rep(" ", 30), collapse = ""), 2)) +
  coord_cartesian(clip = "off") +
  annotation_custom(grid::roundrectGrob(x = -0.06, y = 0.27,
                    width = 0.07, height = 0.05, r = grid::unit(2, "mm"), 
                    gp = grid::gpar(fill = "blue"))) +
  annotation_custom(grid::roundrectGrob(x = -0.06, y = 0.73,
                    width = 0.07, height = 0.05, r = grid::unit(2, "mm"), 
                    gp = grid::gpar(fill = "red")))