Suggestion for `geom_vector()` key glyphs

[teunbrand]

When using geom_vector() + scale_mag(), you get a legend displaying the magnitude. This is fine, but I can imagine maintaining guide_vector() might be a bit of a hassle (certainly given recent guide kerfuffle).

library(ggplot2)
library(metR)

ggplot(seals, aes(long, lat)) +
  geom_vector(aes(dx = delta_long, dy = delta_lat), skip = 2) +
  scale_mag()
#> Warning: The S3 guide system was deprecated in ggplot2 3.5.0.
#> ℹ It has been replaced by a ggproto system that can be extended.
#> This warning is displayed once every 8 hours.
#> Call `lifecycle::last_lifecycle_warnings()` to see where this warning was
#> generated.

So the suggestion I'd like to put out there, is that you can use a gimmick in ggplot2 >3.5.0 that lets you assign size to legend glyphs. That way, you can just use regular legends and you're able to display multiple breaks if needed. As a quick and dirty demo, here is how that might work:

draw_key_vector <- function(data, params, size) {
  if (is.null(data$linetype)) {
    data$linetype <- 0
  } else {
    data$linetype[is.na(data$linetype)] <- 0
  }

  `%||%` <- function(a, b) if (is.null(a)) b else a
    
  grob <- grid::segmentsGrob(
    x0 = 0.1, y0 = 0.5, x1 = unit(0.1, "npc") + unit(data$mag, "cm"), y1 = 0.5,
    gp = grid::gpar(
      col = alpha(data$colour %||% data$fill %||% "black", data$alpha),
      fill = alpha(params$arrow.fill %||% data$colour %||% data$fill %||% "black", data$alpha),
      lwd = (data$linewidth %||% 0.5) * .pt,
      lty = data$linetype %||% 1,
      lineend = params$lineend %||% "butt"
    ),
    arrow = params$arrow
  )
  # Magick number is 1.25 because we like to span 80% of the width with the segment, so the
  # total width is 1 / 0.8 * size == 1.25 * size
  attr(grob, "width") <- 1.25 * data$mag # assumes cm
  grob
  
}

ggplot(seals, aes(long, lat)) +
    geom_vector(aes(dx = delta_long, dy = delta_lat), 
                skip = 2, key_glyph = draw_key_vector) +
    continuous_scale("mag", palette = scales::identity_pal())

^{Created on 2024-01-19 with reprex v2.1.0}

No need to implement this of course, but as I've been rummaging around metR this idea struck me and thought I'd share.