Fix linewidth of draw_key_polygon() (#5815)

* `draw_key_polygon()` uses actual linewidth

* add news bullet

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* The size of the `draw_key_polygon()` glyph now reflects the `linewidth` 
+ aesthetic (#4852).
 * New function `complete_theme()` to replicate how themes are handled during
  plot building (#5801).
 * Special getter and setter functions have been renamed for consistency, allowing

R/legend-draw.R

@@ -71,9 +71,9 @@ draw_key_polygon <- function(data, params, size) {
  data$linewidth <- 0.5
  }
 
- lwd <- min(data$linewidth, min(size) / 4)
+ lwd <- data$linewidth
 
- rectGrob(
+ grob <- rectGrob(
  width = unit(1, "npc") - unit(lwd, "mm"),
  height = unit(1, "npc") - unit(lwd, "mm"),
  gp = ggpar(
@@ -84,6 +84,12 @@ draw_key_polygon <- function(data, params, size) {
  linejoin = params$linejoin %||% "mitre",
  lineend = params$lineend %||% "butt"
  ))
+
+ # Magic number is 5 because we convert mm to cm (divide by 10) but we
+ # draw two lines in each direction (times 2)
+ attr(grob, "width") <- lwd / 5
+ attr(grob, "height") <- lwd / 5
+ grob
 }
 
 #' @export