Robust position_dodge(preserve = "single") (tidyverse#5928)

* count groups per position instead of position per group

* add news bullet

Author: teunbrand

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* `position_dodge(preserve = "single")` now handles multi-row geoms better,
+  such as `geom_violin()` (@teunbrand based on @clauswilke's work, #2801).
 * `position_jitterdodge()` now dodges by `group` (@teunbrand, #3656)
 * The `arrow.fill` parameter is now applied to more line-based functions: 
   `geom_path()`, `geom_line()`, `geom_step()` `geom_function()`, line 

R/position-dodge.R

@@ -111,9 +111,9 @@ PositionDodge <- ggproto("PositionDodge", Position,
     if (identical(self$preserve, "total")) {
       n <- NULL
     } else {
-      panels <- unname(split(data, data$PANEL))
-      ns <- vapply(panels, function(panel) max(table(panel$xmin)), double(1))
-      n <- max(ns)
+      n <- vec_unique(data[c("group", "PANEL", "xmin")])
+      n <- vec_group_id(n[c("PANEL", "xmin")])
+      n <- max(tabulate(n, attr(n, "n")))
     }
 
     list(