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

NEWS.md

@@ -1,4 +1,13 @@
 # ggplot2 3.1.0.9000
+
+* `geom_hline()`, `geom_vline()` & `geom_abline()` now throw a warning if the user supplies both an xintercept, yintercept or slope value & a mapping such as this:
+
+  ```R
+  geom_hline(yintercept = 3, aes(colour = colour))
+
+  ```
+
+  The above will cause the user supplied mapping to be ignored/overwritten and as users might resaonably expect the above to work given that other geoms do  not have this behaviour, they will now be warned about mapping being overwritten. (@RichardJActon, #2950)
 
 * `scale_shape_identity()` now works correctly with `guide = "legend"` (@malcolmbarrett, #3029)
 

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 `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
     n_slopes <- max(length(slope), length(intercept))

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 <- new_data_frame(list(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 <- new_data_frame(list(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 "
+  )
+
+  expect_warning(
+    geom_abline(intercept = 3, slope = 0.5, aes(colour = colour)),
+    "Using "
+  )
+
+  expect_warning(
+    geom_abline(slope=0.5, aes(colour = colour)),
+    "Using "
+  )
+})