Warn when a supplied mapping is going to be overwritten by geom_hline / vline / abline

R/geom-abline.r

@@ -80,6 +80,15 @@ geom_abline <- function(mapping = NULL, data = NULL,
     intercept <- 0
   }
 
+  # Warn if supplied mapping is going to be overwritten
+  if ((!missing(slope) || !missing(intercept)) && !missing(mapping)) {
+    warning(paste0("Using both `intercept` and/or `slope` with `mapping` may not have the desired result as mapping is overwritten if either of these is specified\n",
+                   "  Consider placing `intercept` / `slope` inside your `aes()` call.\n",
+                   "  e.g. `aes(intercept=2,colour=colour)`"
+            )
+    )
+  }
+
   # Act like an annotation
   if (!missing(slope) || !missing(intercept)) {
     if (missing(slope)) slope <- 1

R/geom-hline.r

@@ -9,6 +9,15 @@ geom_hline <- function(mapping = NULL, data = NULL,
                        na.rm = FALSE,
                        show.legend = NA) {
 
+  # Warn if supplied mapping is going to be overwritten
+  if (!missing(yintercept) && !missing(mapping)) {
+    warning(paste0("Using both `yintercept` and `mapping` may not have the desired result as mapping is overwritten if `yintercept` is specified\n",
+                   "  Consider placing `yintercept` inside your `aes()` call.\n",
+                   "  e.g. `aes(yintercept=2,colour=colour)`"
+            )
+    )
+  }
+
   # Act like an annotation
   if (!missing(yintercept)) {
     data <- data.frame(yintercept = yintercept)

R/geom-vline.r

@@ -9,6 +9,15 @@ geom_vline <- function(mapping = NULL, data = NULL,
                        na.rm = FALSE,
                        show.legend = NA) {
 
+  # Warn if supplied mapping is going to be overwritten
+  if (!missing(xintercept) && !missing(mapping)) {
+    warning(paste0("Using both `xintercept` and `mapping` may not have the desired result as mapping is overwritten if `xintercept` is specified\n",
+                   "  Consider placing `xintercept` inside your `aes()` call.\n",
+                   "  e.g. `aes(xintercept=2,colour=colour)`"
+            )
+    )
+  }
+
   # Act like an annotation
   if (!missing(xintercept)) {
     data <- data.frame(xintercept = xintercept)

tests/testthat/test-geom-hline-vline-abline.R

@@ -43,3 +43,38 @@ test_that("curved lines in map projections", {
     nzmap + coord_map(projection = 'azequalarea', orientation = c(-36.92, 174.6, 0))
   )
 })
+
+# Warning tests ------------------------------------------------------------
+
+test_that("Warning if a supplied mapping is going to be overwritten",{
+  dat <- data.frame(x = LETTERS[1:5], y = 1:5,colour = c(TRUE,TRUE,rep(FALSE,3)))
+  plot <- ggplot(dat, aes(x, y)) +
+    geom_col(width = 1) +
+    geom_point() +
+    facet_wrap(~colour)
+
+  expect_warning(
+    plot + geom_vline(xintercept = 3, aes(colour = colour)),
+    "Using both"
+  )
+
+  expect_warning(
+    plot + geom_hline(yintercept = 3, aes(colour = colour)),
+    "Using both"
+  )
+
+  expect_warning(
+    plot + geom_abline(intercept = 3, aes(colour = colour)),
+    "Using both"
+  )
+
+  expect_warning(
+    plot + geom_abline(intercept = 3,slope=0.5, aes(colour = colour)),
+    "Using both"
+  )
+
+  expect_warning(
+    plot + geom_abline(slope=0.5, aes(colour = colour)),
+    "Using both"
+  )
+})