A way to ensure limits are included in the breaks on a scale and get labeled with ≤ and ≥ when using oob_squish()

breaks_limits.R

library(ggplot2)
library(scales)
breaks_limits <- function (n = 5, tol = 0.1, min = TRUE, max = TRUE, ...) 
{
  n_default <- n
  scales:::force_all(n, tol, min, max, ...)
  function(x, n = n_default) {
    breaks <- pretty(x, n, ...)
    
    #force limits to be included and remove breaks outside of limits
    if (isTRUE(min)) {
      breaks <- c(x[1], breaks)
    }
    if (isTRUE(max)) {
      breaks <- c(x[2], breaks)
    }
    breaks <- unique(sort(breaks))
    breaks <- breaks[breaks>=x[1] & breaks<=x[2]]
    
    #remove breaks too close to limits that they are likely to overlap
    scl_br <- (breaks - min(breaks)) / diff(range(breaks)) #or diff(x)
    if (isTRUE(min) & abs(scl_br[1] - scl_br[2]) < tol) {
      breaks <- breaks[-2]
    }
    if (isTRUE(max) & abs(scl_br[length(scl_br)] - scl_br[length(scl_br) - 1]) < tol) {
      breaks <- breaks[-(length(breaks)-1)]
    }
    labels <- as.character(breaks)
    if (isTRUE(min)) {
      labels[1] <- paste0("≤ ", labels[1])
    }
    if (isTRUE(max)) {
      labels[length(labels)] <- paste0("≥ ", labels[length(labels)])
    }
    names(breaks) <- labels
    breaks
  }
}

ggplot(faithful, aes(waiting, eruptions, color = eruptions)) +
  geom_point() +
  scale_y_continuous(
    limits = c(NA, 4.2),
    breaks = breaks_limits(min = FALSE),
    labels = \(x) names(x),
    oob = oob_squish,
    expand = c(0,0)
  ) +
  scale_color_continuous(
    limits = c(NA, 4.2),
    breaks = breaks_limits(min = FALSE),
    labels = \(x) names(x),
    oob = oob_squish
  ) +
  coord_cartesian(clip = "off")

Doesn't quite work for positional scales because "Note that for position scales, limits are provided after scale expansion." (from breaks arg of scale_y_continuous()).

More discussion on this here: r-lib/scales#441