Clarified needed input to stat_contour

Fixes #1577

Author: hadley

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 2.1.0.9000 
 
+* Clarified needed input to `geom_contour()` (#1577).
+
 * Import tibble. This ensures that all built-in datasets print compactly
   even if you haven't explicitly loaded tibble or dplyr (#1677).
 

R/geom-contour.r

@@ -1,5 +1,9 @@
 #' Display contours of a 3d surface in 2d.
 #'
+#' To be a valid surface, the most only be a single combination of each
+#' unique \code{x} and \code{y} aesthetics. Contouring tends to work best
+#' when \code{x} and \code{y} form a (roughly) evenly spaced grid.
+#'
 #' @section Aesthetics:
 #' \Sexpr[results=rd,stage=build]{ggplot2:::rd_aesthetics("geom", "contour")}
 #'

R/stat-contour.r

@@ -66,6 +66,11 @@ StatContour <- ggproto("StatContour", Stat,
 contour_lines <- function(data, breaks, complete = FALSE) {
   z <- tapply(data$z, data[c("x", "y")], identity)
 
+  if (is.list(z)) {
+    stop("Contour requires single `z` at each combination of `x` and `y`.",
+      call. = FALSE)
+  }
+
   cl <- grDevices::contourLines(
     x = sort(unique(data$x)), y = sort(unique(data$y)), z = z,
     levels = breaks)