ggplot stop geom_segment showing over geom_boxplot in legend

Question

I am trying to get something similar to Glen_b's answer to this question. Basically, I want an arrow at the top of my boxplot to indicate that there are more outliers out of scale using ggplot.

I can get the main part of the plot to look as I'd like, but I am running into problems getting a sensible legend.

I have put together an example:

library(plyr)
library(dplyr)
library(ggplot2)

#create test data frame with extreme outlier
mpg_test <- mpg

mpg_test[1,"hwy"] = 250
pmax = 50
pmin = min(mpg_test$hwy)

outliersabovepmax <- filter(mpg_test, hwy > pmax) %>% mutate(hwy= pmax) 

#basic plot without cropping/adding arrows
p <- ggplot(mpg_test, aes(x = class, y=hwy, colour = class)) + geom_boxplot()
p

I can get the main body of the plot to look as I'd like:

#plot that I want
p2 <- p + 
  geom_segment(data = outliersabovepmax, 
               aes(xend = class, y = hwy-1, yend = hwy+1, 
                   linetype = "Outlier above"), 
               arrow = arrow(), show.legend = T) +
  coord_cartesian(ylim = c(pmin, pmax))

But I'd like the legend to not show the arrow on the colour legend.

I would normally expect to be able to do something along the lines of:

p2 + 
  guides(colour = guide_legend(override.aes = list(linetype = "blank")))

but because the boxplot is also using linetype I just end up with black squares if I do this. I also don't want to set show.legend = F for geom_segment as I want to have the arrow in the legend.

As a bonus I'd like the arrow in the legend to be rotated to face up. But this is not as important.

Plot that I get:

Plot that I'd like (made in paint):

Answer 1

Maybe something like this?

p + 
  # hide legend in actual segment layer
  geom_segment(data = outliersabovepmax, 
               aes(xend = class, y = hwy-1, yend = hwy+1), 
               arrow = arrow(), show.legend = F)+

  # have invisible segment layer that shows legend
  geom_segment(data = outliersabovepmax, 
               aes(xend = class, y = hwy-1, yend = hwy+1, linetype = "Outlier above"), 
               arrow = arrow(), alpha = 0, show.legend = T) +

  # override alpha for linetype legend
  guides(linetype = guide_legend(override.aes = list(alpha = 1))) +

  coord_cartesian(ylim = c(pmin, pmax))

And if you want to change the layout of geom_segment's legend key, you can dig into the underlying code for GeomSegment:

library(grid)
GeomSegment2 <- ggproto("GeomSegment2", GeomSegment,
                        draw_key = function (data, params, size)                         {
                          data$linetype[is.na(data$linetype)] <- 0
                          segmentsGrob(
                            # vertical instead of horizontal line
                            0.5, 0.1, 0.5, 0.9, #0.1, 0.5, 0.9, 0.5, 
                            gp = gpar(col = alpha(data$colour, data$alpha), 
                                      lwd = data$size * .pt, 
                                      lty = data$linetype, 
                                      lineend = "butt"), 
                            arrow = params$arrow)
                        })
geom_segment2 <- function (mapping = NULL, data = NULL, stat = "identity", 
                           position = "identity", ..., arrow = NULL, arrow.fill = NULL, 
                           lineend = "butt", linejoin = "round", na.rm = FALSE, 
                           show.legend = NA, inherit.aes = TRUE) {
  layer(data = data, mapping = mapping, stat = stat, geom = GeomSegment2, 
        position = position, show.legend = show.legend, inherit.aes = inherit.aes, 
        params = list(arrow = arrow, arrow.fill = arrow.fill, 
                      lineend = lineend, linejoin = linejoin, na.rm = na.rm, 
                      ...))
}

Usage:

p + 
  # hide legend in actual segment layer
  geom_segment(data = outliersabovepmax, 
               aes(xend = class, y = hwy-1, yend = hwy+1), 
               arrow = arrow(), show.legend = F)+

  # have invisible segment layer that shows legend
  geom_segment2(data = outliersabovepmax, 
               aes(xend = class, y = hwy-1, yend = hwy+1, linetype = "Outlier above"), 
               arrow = arrow(), alpha = 0, show.legend = T) +

  # override alpha for linetype legend
  guides(linetype = guide_legend(override.aes = list(alpha = 1))) +

  coord_cartesian(ylim = c(pmin, pmax))