ggplot2 hack: annotate a single axis-label and preserve intendation

single-axis-label-annotated.R

library(tidyverse)

annotation <-"HP"

# identify max char length of y-label
ymax_chr <- nchar(as.integer(max(mtcars$hp)))

ggplot(mtcars, aes(x = mpg, y = hp)) +
  geom_point(color = "black", alpha = 0.5) +
  scale_y_continuous(
    # 1. explicitly define the axis breaks
    breaks = seq(0,350,50), # needed to ensure that the label we want to extend is shown, also see coord_cartesian()
    labels = function(x) {
      labels <- x # vector of ALL y-axis labels
      # 2. optional, for hjust = 0 approach: left-pad all labels that'd be shorter than the max label value
      # labels[-length(labels)] <- sprintf(fmt = paste0("%0",ymax_chr,"s"), labels[-length(labels)])
      # 3. right-pad all but the last item with xx em (depends on width of annotation string)
      ## see https://jkorpela.fi/chars/spaces.html for other unicode whitespace elements / widths
      labels[-length(labels)] <- paste0(labels[-length(labels)], "\U2006\U2001\U2000")# "___" ) # 1/6em + 1em + 0.5em
      # 4. add annotation to to the last label value
      labels[length(labels)] <- paste0(labels[length(labels)], " ", annotation)
      return(labels)
    }
  ) +
  coord_cartesian(ylim = c(0,350)) + # helps if you need to ensure that the desired y label is shown
  labs(y = NULL) + # want we want to make obsolete, after all
  theme_minimal() +
  theme(
    ## axis.text.y = ggtext::element_markdown( 
    ## interpretes hjust differently, as of 2021-10-21 has trouble with <pre>)
    axis.text.y = element_text(
      hjust = 1,
      margin = margin(r = -0.4, unit = "cm"),
      # fill = "white" # works only for ggtext afaik
    )
  )

Caution: stable with svg(), png() and ragg::agg_png(); doesn't 100% work (for me) with svglite(). svglite ignores the whitespace padding hack