Extent the range of geom_abline() outside the panel range (#4422)

Author: thomasp85

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* Extent the range of the line created by `geom_abline()` so that line ending
+  is not visible for large linewidths (@thomasp85, #4024)
 
 * Make sure that default labels from default mappings doesn't overwrite default
   labels from explicit mappings (@thomasp85, #2406)

R/geom-abline.r

@@ -127,6 +127,12 @@ GeomAbline <- ggproto("GeomAbline", Geom,
   draw_panel = function(data, panel_params, coord) {
     ranges <- coord$backtransform_range(panel_params)
 
+    if (coord$clip == "on" && coord$is_linear()) {
+      # Ensure the line extends well outside the panel to avoid visible line
+      # ending for thick lines
+      ranges$x <- ranges$x + c(-1, 1) * diff(ranges$x)
+    }
+
     data$x    <- ranges$x[1]
     data$xend <- ranges$x[2]
     data$y    <- ranges$x[1] * data$slope + data$intercept