Expose xseq in stat_smooth() (#5466)

* Expose `xseq` in `stat_smooth()`

* Take Gina's suggestions

R/stat-smooth.R

@@ -23,6 +23,9 @@
 #' @param fullrange If `TRUE`, the smoothing line gets expanded to the range of the plot,
 #'   potentially beyond the data. This does not extend the line into any additional padding
 #'   created by `expansion`.
+#' @param xseq A numeric vector of values at which the smoother is evaluated.
+#'   When `NULL` (default), `xseq` is internally evaluated as a sequence of `n`
+#'   equally spaced points for continuous data.
 #' @param level Level of confidence interval to use (0.95 by default).
 #' @param span Controls the amount of smoothing for the default loess smoother.
 #'   Smaller numbers produce wigglier lines, larger numbers produce smoother
@@ -52,6 +55,7 @@ stat_smooth <- function(mapping = NULL, data = NULL,
                         n = 80,
                         span = 0.75,
                         fullrange = FALSE,
+                        xseq = NULL,
                         level = 0.95,
                         method.args = list(),
                         na.rm = FALSE,
@@ -77,6 +81,7 @@ stat_smooth <- function(mapping = NULL, data = NULL,
       orientation = orientation,
       method.args = method.args,
       span = span,
+      xseq = xseq,
       ...
     )
   )