Make sure jitter is only calculated once

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 (development version)
 
+* Fix a bug in `position_jitter()` where different jitters would be applied to 
+  different position aesthetics of the same axis (@thomasp85, #2941)
+
 * `coord_sf()` now has an argument `default_crs` that specifies the coordinate
   reference system (CRS) for non-sf layers and scale/coord limits. This argument
   defaults to the World Geodetic System 1984 (WGS84), which means x and y positions

R/position-jitter.r

@@ -76,6 +76,17 @@ PositionJitter <- ggproto("PositionJitter", Position,
     trans_x <- if (params$width > 0) function(x) jitter(x, amount = params$width)
     trans_y <- if (params$height > 0) function(x) jitter(x, amount = params$height)
 
-    with_seed_null(params$seed, transform_position(data, trans_x, trans_y))
+    # Make sure x and y jitter is only calculated once for all position aesthetics
+    x_aes <- intersect(names(data), ggplot_global$x_aes)
+    x <- if (length(x_aes) == 0) 0 else data[[x_aes[1]]]
+    y_aes <- intersect(names(data), ggplot_global$y_aes)
+    y <- if (length(y_aes) == 0) 0 else data[[y_aes[1]]]
+    dummy_data <- new_data_frame(list(x = x, y = y), nrow(data))
+    fixed_jitter <- with_seed_null(params$seed, transform_position(dummy_data, trans_x, trans_y))
+    x_jit <- fixed_jitter$x - x
+    y_jit <- fixed_jitter$y - y
+
+    # Apply jitter
+    transform_position(data, function(x) x + x_jit, function(x) x + y_jit)
   }
 )