Feat/issue 4899 simplify alignment for column geoms

NEWS.md

@@ -1,5 +1,10 @@
 # ggplot2 (development version)
 
+* `geom_col()` and `geom_bar()` gain a new `just` argument. This is set to `0.5`
+  by default; use `just = 0`/`just = 1` to place columns on the left/right
+  of the axis breaks.
+  (@wurli, #4899)
+
 * Fix a bug in `position_jitter()` where infinity values were dropped (@javlon,
   #4790).
 
@@ -14,6 +19,7 @@
   columns were dropped and warns about this. If stats intend to drop
   data columns they can declare them in the new field `dropped_aes`.
   (@clauswilke, #3250)
+
 * Added `stat_align()` to align data without common x-coordinates prior to
   stacking. This is now the default stat for `geom_area()` (@thomasp85, #4850)
 

R/geom-bar.r

@@ -40,7 +40,13 @@
 #' automatically determines the orientation from the aesthetic mapping. In the
 #' rare event that this fails it can be given explicitly by setting `orientation`
 #' to either `"x"` or `"y"`. See the *Orientation* section for more detail.
-#' @param width Bar width. By default, set to 90% of the resolution of the data.
+#' @param just Adjustment for column placement. Set to `0.5` by default, meaning
+#'   that columns will be centered about axis breaks. Set to `0` or `1` to place
+#'   columns to the left/right of axis breaks. Note that this argument may have
+#'   unintended behaviour when used with alternative positions, e.g.
+#'   `position_dodge()`.
+#' @param width Bar width. By default, set to 90% of the [resolution()] of the
+#'   data.
 #' @param geom,stat Override the default connection between `geom_bar()` and
 #'   `stat_count()`.
 #' @examples
@@ -80,9 +86,17 @@
 #' ggplot(df, aes(x)) + geom_bar()
 #' # cf. a histogram of the same data
 #' ggplot(df, aes(x)) + geom_histogram(binwidth = 0.5)
+#'
+#' # Use `just` to control how columns are aligned with axis breaks:
+#' df <- data.frame(x = as.Date(c("2020-01-01", "2020-02-01")), y = 1:2)
+#' # Columns centered on the first day of the month
+#' ggplot(df, aes(x, y)) + geom_col(just = 0.5)
+#' # Columns begin on the first day of the month
+#' ggplot(df, aes(x, y)) + geom_col(just = 1)
 geom_bar <- function(mapping = NULL, data = NULL,
                      stat = "count", position = "stack",
                      ...,
+                     just = 0.5,
                      width = NULL,
                      na.rm = FALSE,
                      orientation = NA,
@@ -97,6 +111,7 @@ geom_bar <- function(mapping = NULL, data = NULL,
     show.legend = show.legend,
     inherit.aes = inherit.aes,
     params = list2(
+      just = just,
       width = width,
       na.rm = na.rm,
       orientation = orientation,
@@ -123,16 +138,18 @@ GeomBar <- ggproto("GeomBar", GeomRect,
     params
   },
 
-  extra_params = c("na.rm", "orientation"),
+  extra_params = c("just", "na.rm", "orientation"),
 
   setup_data = function(data, params) {
     data$flipped_aes <- params$flipped_aes
     data <- flip_data(data, params$flipped_aes)
     data$width <- data$width %||%
       params$width %||% (resolution(data$x, FALSE) * 0.9)
+    data$just <- params$just %||% 0.5
     data <- transform(data,
       ymin = pmin(y, 0), ymax = pmax(y, 0),
-      xmin = x - width / 2, xmax = x + width / 2, width = NULL
+      xmin = x - width * (1 - just), xmax = x + width * just,
+      width = NULL, just = NULL
     )
     flip_data(data, params$flipped_aes)
   },

R/geom-col.r

@@ -3,6 +3,7 @@
 geom_col <- function(mapping = NULL, data = NULL,
                      position = "stack",
                      ...,
+                     just = 0.5,
                      width = NULL,
                      na.rm = FALSE,
                      show.legend = NA,
@@ -17,6 +18,7 @@ geom_col <- function(mapping = NULL, data = NULL,
     show.legend = show.legend,
     inherit.aes = inherit.aes,
     params = list2(
+      just = just,
       width = width,
       na.rm = na.rm,
       ...
@@ -42,16 +44,19 @@ GeomCol <- ggproto("GeomCol", GeomRect,
     params
   },
 
-  extra_params = c("na.rm", "orientation"),
+  extra_params = c("just", "na.rm", "orientation"),
 
   setup_data = function(data, params) {
     data$flipped_aes <- params$flipped_aes
     data <- flip_data(data, params$flipped_aes)
     data$width <- data$width %||%
       params$width %||% (resolution(data$x, FALSE) * 0.9)
-    data <- transform(data,
-                      ymin = pmin(y, 0), ymax = pmax(y, 0),
-                      xmin = x - width / 2, xmax = x + width / 2, width = NULL
+    data$just <- params$just %||% 0.5
+    data <- transform(
+      data,
+      ymin = pmin(y, 0), ymax = pmax(y, 0),
+      xmin = x - width * (1 - just), xmax = x + width * just,
+      width = NULL, just = NULL
     )
     flip_data(data, params$flipped_aes)
   },

tests/testthat/test-geom-col.R

@@ -28,3 +28,22 @@ test_that("geom_col works in both directions", {
   y$flipped_aes <- NULL
   expect_identical(x, flip_data(y, TRUE)[,names(x)])
 })
+
+test_that("geom_col supports alignment of columns", {
+  dat <- data_frame(x = c("a", "b"), y = c(1.2, 2.5))
+
+  p <- ggplot(dat, aes(x, y)) + geom_col(just = 0.5)
+  y <- layer_data(p)
+  expect_equal(as.numeric(y$xmin), c(0.55, 1.55))
+  expect_equal(as.numeric(y$xmax), c(1.45, 2.45))
+
+  p <- ggplot(dat, aes(x, y)) + geom_col(just = 0.0)
+  y <- layer_data(p)
+  expect_equal(as.numeric(y$xmin), c(0.1, 1.1))
+  expect_equal(as.numeric(y$xmax), c(1.0, 2.0))
+
+  p <- ggplot(dat, aes(x, y)) + geom_col(just = 1.0)
+  y <- layer_data(p)
+  expect_equal(as.numeric(y$xmin), c(1.0, 2.0))
+  expect_equal(as.numeric(y$xmax), c(1.9, 2.9))
+})