Clip qq line (#6220)

* `geom/stat_qq_line()` use `stat = "abline"`

* document new computed variables

* add news bullet

Author: teunbrand

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 (development version)
 
+* New default `geom_qq_line(geom = "abline")` for better clipping in the 
+  vertical direction. In addition, `slope` and `intercept` are new computed
+  variables in `stat_qq_line()` (@teunbrand, #6087).
 * Position adjustments can now have auxiliary aesthetics (@teunbrand).
     * `position_nudge()` gains `nudge_x` and `nudge_y` aesthetics (#3026, #5445).
     * `position_dodge()` gains `order` aesthetic (#3022, #3345)

R/stat-qq-line.R

@@ -6,7 +6,7 @@
 #'   the data
 geom_qq_line <- function(mapping = NULL,
                          data = NULL,
-                         geom = "path",
+                         geom = "abline",
                          position = "identity",
                          ...,
                          distribution = stats::qnorm,
@@ -86,6 +86,9 @@ StatQqLine <- ggproto("StatQqLine", Stat,
       x <- range(theoretical)
     }
 
-    data_frame0(x = x, y = slope * x + intercept)
+    data_frame0(
+      x = x, y = slope * x + intercept,
+      slope = slope, intercept = intercept
+    )
   }
 )

R/stat-qq.R

@@ -22,7 +22,9 @@
 #'   x = "x-coordinates of the endpoints of the line segment connecting the
 #'   points at the chosen quantiles of the theoretical and the sample
 #'   distributions.",
-#'   y = "y-coordinates of the endpoints."
+#'   y = "y-coordinates of the endpoints.",
+#'   slope     = "Amount of change in `y` across 1 unit of `x`.",
+#'   intercept = "Value of `y` at `x == 0`."
 #' )
 #'
 #' @export