Optional segment end (#5154)

* Make one of xend/yend optional

* Add NEWS bullet

* Reoxygenate

Author: teunbrand

NEWS.md

@@ -1,5 +1,10 @@
 # ggplot2 (development version)
 
+* You can now omit either `xend` or `yend` from `geom_segment()` as only one
+  of these is now required. If one is missing, it will be filled from the `x`
+  and `y` aesthetics respectively. This makes drawing horizontal or vertical
+  segments a little bit more convenient (@teunbrand, #5140).
+  
 * New `plot.tag.location` in `theme()` can control placement of the plot tag
   in the `"margin"`, `"plot"` or the new `"panel"` option (#4297).
 

R/geom-segment.R

@@ -102,11 +102,13 @@ geom_segment <- function(mapping = NULL, data = NULL,
 #' @usage NULL
 #' @export
 GeomSegment <- ggproto("GeomSegment", Geom,
-  required_aes = c("x", "y", "xend", "yend"),
+  required_aes = c("x", "y", "xend|yend"),
   non_missing_aes = c("linetype", "linewidth", "shape"),
   default_aes = aes(colour = "black", linewidth = 0.5, linetype = 1, alpha = NA),
   draw_panel = function(self, data, panel_params, coord, arrow = NULL, arrow.fill = NULL,
                         lineend = "butt", linejoin = "round", na.rm = FALSE) {
+    data$xend <- data$xend %||% data$x
+    data$yend <- data$yend %||% data$y
     data <- check_linewidth(data, snake_class(self))
     data <- remove_missing(data, na.rm = na.rm,
       c("x", "y", "xend", "yend", "linetype", "linewidth", "shape"),