Added linejoin parameter to geom_segment.

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 2.2.1.9000
 
+* `geom_segment` now also takes a `linejoin` parameter. This allows more control over the appearance of the segments, which is especially useful for plotting thick arrows (@Ax3man, #774).
+
 * Theme elements can now be subclassed. Add a `merge_element` method to control
   how properties are inherited from parent element. Add `element_grob` method
   to define how elements are rendered into grobs (@thomasp85, #1981).

R/geom-segment.r

@@ -15,6 +15,7 @@
 #' @inheritParams geom_point
 #' @param arrow specification for arrow heads, as created by arrow().
 #' @param lineend Line end style (round, butt, square).
+#' @param linejoin Line join style (round, mitre, bevel).
 #' @seealso [geom_path()] and [geom_line()] for multi-
 #'   segment lines and paths.
 #' @seealso [geom_spoke()] for a segment parameterised by a location
@@ -42,6 +43,21 @@
 #'     arrow = arrow(length = unit(0.1,"cm"))) +
 #'   borders("state")
 #'
+#' # Use lineend and linejoin to change the style of the segments
+#' df2 <- expand.grid(
+#'   lineend = c('round', 'butt', 'square'),
+#'   linejoin = c('round', 'mitre', 'bevel'),
+#'   stringsAsFactors = FALSE
+#' )
+#' df2 <- data.frame(df2, y = 1:9)
+#' ggplot(df2, aes(x = 1, y = y, xend = 2, yend = y, label = paste(lineend, linejoin))) +
+#'   geom_segment(
+#'      lineend = df2$lineend, linejoin = df2$linejoin,
+#'      size = 3, arrow = arrow(length = unit(0.3, "inches"))
+#'   ) +
+#'   geom_text(hjust = 'outside', nudge_x = -0.2) +
+#'   xlim(0.5, 2)
+#'
 #' # You can also use geom_segment to recreate plot(type = "h") :
 #' counts <- as.data.frame(table(x = rpois(100,5)))
 #' counts$x <- as.numeric(as.character(counts$x))
@@ -54,6 +70,7 @@ geom_segment <- function(mapping = NULL, data = NULL,
                          ...,
                          arrow = NULL,
                          lineend = "butt",
+                         linejoin = "round",
                          na.rm = FALSE,
                          show.legend = NA,
                          inherit.aes = TRUE) {
@@ -68,6 +85,7 @@ geom_segment <- function(mapping = NULL, data = NULL,
     params = list(
       arrow = arrow,
       lineend = lineend,
+      linejoin = linejoin,
       na.rm = na.rm,
       ...
     )
@@ -84,7 +102,7 @@ GeomSegment <- ggproto("GeomSegment", Geom,
   default_aes = aes(colour = "black", size = 0.5, linetype = 1, alpha = NA),
 
   draw_panel = function(data, panel_params, coord, arrow = NULL,
-                        lineend = "butt", na.rm = FALSE) {
+                        lineend = "butt", linejoin = "round", na.rm = FALSE) {
 
     data <- remove_missing(data, na.rm = na.rm,
       c("x", "y", "xend", "yend", "linetype", "size", "shape"),
@@ -100,7 +118,8 @@ GeomSegment <- ggproto("GeomSegment", Geom,
           fill = alpha(coord$colour, coord$alpha),
           lwd = coord$size * .pt,
           lty = coord$linetype,
-          lineend = lineend
+          lineend = lineend,
+          linejoin = linejoin
         ),
         arrow = arrow
       ))