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

R/geom-abline.r

@@ -82,6 +82,16 @@ geom_abline <- function(mapping = NULL, data = NULL,
 
   # Act like an annotation
   if (!missing(slope) || !missing(intercept)) {
+
+    # Warn if supplied mapping is going to be overwritten
+    if (!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"
+              )
+      )
+    }
+
     if (missing(slope)) slope <- 1
     if (missing(intercept)) intercept <- 0
 

R/geom-hline.r

@@ -11,6 +11,14 @@ geom_hline <- function(mapping = NULL, data = NULL,
 
   # Act like an annotation
   if (!missing(yintercept)) {
+    # Warn if supplied mapping is going to be overwritten
+    if (!missing(mapping)) {
+      warning(paste0("Using both `yintercept` and `mapping` may not have the",
+                     " desired result as mapping is overwritten if",
+                     " `yintercept` is specified\n")
+              )
+    }
+
     data <- data.frame(yintercept = yintercept)
     mapping <- aes(yintercept = yintercept)
     show.legend <- FALSE

R/geom-vline.r

@@ -11,6 +11,14 @@ geom_vline <- function(mapping = NULL, data = NULL,
 
   # Act like an annotation
   if (!missing(xintercept)) {
+    # Warn if supplied mapping is going to be overwritten
+    if (!missing(mapping)) {
+      warning(paste0("Using both `xintercept` and `mapping` may not have the",
+                     " desired result as mapping is overwritten if",
+                     " `xintercept` is specified\n"
+              )
+      )
+    }
     data <- data.frame(xintercept = xintercept)
     mapping <- aes(xintercept = xintercept)
     show.legend <- FALSE

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

@@ -43,3 +43,33 @@ 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",{
+
+  expect_warning(
+    geom_vline(xintercept = 3, aes(colour = colour)),
+    "Using both"
+  )
+
+  expect_warning(
+    geom_hline(yintercept = 3, aes(colour = colour)),
+    "Using both"
+  )
+
+  expect_warning(
+    geom_abline(intercept = 3, aes(colour = colour)),
+    "Using both"
+  )
+
+  expect_warning(
+    geom_abline(intercept = 3,slope=0.5, aes(colour = colour)),
+    "Using both"
+  )
+
+  expect_warning(
+    geom_abline(slope=0.5, aes(colour = colour)),
+    "Using both"
+  )
+})