Fix back-transformation of ranges in coords

NEWS.md

@@ -10,6 +10,13 @@
 *   `coord_sf()` now accepts two new parameters, `label_graticule` and `label_axes`,
     that can be used to specify which graticules to label on which side of the plot
     (@clauswilke, #2846).
+
+*   `Coord` objects now have a function `backtransform_range()` that returns the
+    panel range in data coordinates. This change may affect developers of custom coords,
+    who now should implement this function. It may also affect developers of custom
+    geoms that use the `range()` function. In some applications, `backtransform_range()`
+    may be more appropriate.
+    (@clauswilke, #2821).
 
 *   `geom_sf()` now respects `lineend`, `linejoin`, and `linemitre` parameters 
     for lines and polygons (@alistaire47, #2826)

R/coord-.r

@@ -13,12 +13,21 @@
 #'   - `render_bg`: Renders background elements.
 #'   - `render_axis_h`: Renders the horizontal axes.
 #'   - `render_axis_v`: Renders the vertical axes.
+#'   - `backtransform_range(panel_params)`: Extracts the panel range provided
+#'     in `panel_params` (created by `setup_panel_params()`, see below) and
+#'     back-transforms to data coordinates. This back-transformation can be needed
+#'     for coords such as `coord_trans()` where the range in the transformed
+#'     coordinates differs from the range in the untransformed coordinates. Returns
+#'     a list of two ranges, `x` and `y`, and these correspond to the variables
+#'     mapped to the `x` and `y` aesthetics, even for coords such as `coord_flip()`
+#'     where the `x` aesthetic is shown along the y direction and vice versa.
 #'   - `range(panel_params)`: Extracts the panel range provided
 #'     in `panel_params` (created by `setup_panel_params()`, see below) and
-#'     back-transforms to data coordinates. This back-transformation is needed
-#'     for coords such as `coord_flip()`, `coord_polar()`, `coord_trans()` where
-#'     the range in the transformed coordinates differs from the range in the
-#'     untransformed coordinates.
+#'     returns it. Unlike `backtransform_range()`, this function does not perform
+#'     any back-transformation and instead returns final transformed coordinates. Returns
+#'     a list of two ranges, `x` and `y`, and these correspond to the variables
+#'     mapped to the `x` and `y` aesthetics, even for coords such as `coord_flip()`
+#'     where the `x` aesthetic is shown along the y direction and vice versa.
 #'   - `transform`: Transforms x and y coordinates.
 #'   - `distance`: Calculates distance.
 #'   - `is_linear`: Returns `TRUE` if the coordinate system is
@@ -84,11 +93,22 @@ Coord <- ggproto("Coord",
   # transform range given in transformed coordinates
   # back into range in given in (possibly scale-transformed)
   # data coordinates
-  range = function(panel_params) {
+  backtransform_range = function(self, panel_params) {
     warning(
-      "range backtransformation not implemented in this coord; plot may be wrong.",
+      "range backtransformation not implemented in this coord; results may be wrong.",
       call. = FALSE
       )
+    # return result from range function for backwards compatibility
+    # before ggplot2 3.0.1
+    self$range(panel_params)
+  },
+
+  # return range stored in panel_params
+  range = function(panel_params) {
+    warning(
+      "range calculation not implemented in this coord; results may be wrong.",
+      call. = FALSE
+    )
     list(x = panel_params$x.range, y = panel_params$y.range)
   },
 

R/coord-cartesian-.r

@@ -87,6 +87,10 @@ CoordCartesian <- ggproto("CoordCartesian", Coord,
     list(x = panel_params$x.range, y = panel_params$y.range)
   },
 
+  backtransform_range = function(self, panel_params) {
+    self$range(panel_params)
+  },
+
   transform = function(data, panel_params) {
     rescale_x <- function(data) rescale(data, from = panel_params$x.range)
     rescale_y <- function(data) rescale(data, from = panel_params$y.range)

R/coord-flip.r

@@ -44,7 +44,13 @@ CoordFlip <- ggproto("CoordFlip", CoordCartesian,
     CoordCartesian$transform(data, panel_params)
   },
 
+  backtransform_range = function(self, panel_params) {
+    self$range(panel_params)
+  },
+
   range = function(panel_params) {
+    # summarise_layout() expects the original x and y ranges here,
+    # not the ones we would get after flipping the axes
     list(x = panel_params$y.range, y = panel_params$x.range)
   },
 

R/coord-map.r

@@ -119,11 +119,19 @@ CoordMap <- ggproto("CoordMap", Coord,
     out
   },
 
-  range = function(panel_params) {
+  backtransform_range = function(panel_params) {
     # range is stored in data coordinates and doesn't have to be back-transformed
     list(x = panel_params$x.range, y = panel_params$y.range)
   },
 
+  range = function(panel_params) {
+    # Range in projected coordinates:
+    #   list(x = panel_params$x.proj, y = panel_params$y.proj)
+    # However, coord_map() does never really work with transformed coordinates,
+    # so return unprojected data coordinates here
+    list(x = panel_params$x.range, y = panel_params$y.range)
+  },
+
   distance = function(x, y, panel_params) {
     max_dist <- dist_central_angle(panel_params$x.range, panel_params$y.range)
     dist_central_angle(x, y) / max_dist

R/coord-munch.r

@@ -15,7 +15,7 @@ coord_munch <- function(coord, data, range, segment_length = 0.01) {
   if (coord$is_linear()) return(coord$transform(data, range))
 
   # range has theta and r values; get corresponding x and y values
-  ranges <- coord$range(range)
+  ranges <- coord$backtransform_range(range)
 
   # Convert any infinite locations into max/min
   # Only need to work with x and y because for munching, those are the

R/coord-polar.r

@@ -90,7 +90,13 @@ CoordPolar <- ggproto("CoordPolar", Coord,
     dist_polar(r, theta)
   },
 
+  backtransform_range = function(self, panel_params) {
+    self$range(panel_params)
+  },
+
   range = function(self, panel_params) {
+    # summarise_layout() expects that the x and y ranges here
+    # match the setting from self$theta and self$r
     setNames(
       list(panel_params$theta.range, panel_params$r.range),
       c(self$theta, self$r)

R/coord-transform.r

@@ -119,13 +119,20 @@ CoordTrans <- ggproto("CoordTrans", Coord,
     dist_euclidean(self$trans$x$transform(x), self$trans$y$transform(y)) / max_dist
   },
 
-  range = function(self, panel_params) {
+  backtransform_range = function(self, panel_params) {
     list(
       x = self$trans$x$inverse(panel_params$x.range),
       y = self$trans$y$inverse(panel_params$y.range)
     )
   },
 
+  range = function(self, panel_params) {
+    list(
+      x = panel_params$x.range,
+      y = panel_params$y.range
+    )
+  },
+
   transform = function(self, data, panel_params) {
     trans_x <- function(data) transform_value(self$trans$x, data, panel_params$x.range)
     trans_y <- function(data) transform_value(self$trans$y, data, panel_params$y.range)

R/geom-abline.r

@@ -111,7 +111,7 @@ geom_abline <- function(mapping = NULL, data = NULL,
 #' @export
 GeomAbline <- ggproto("GeomAbline", Geom,
   draw_panel = function(data, panel_params, coord) {
-    ranges <- coord$range(panel_params)
+    ranges <- coord$backtransform_range(panel_params)
 
     data$x    <- ranges$x[1]
     data$xend <- ranges$x[2]

R/geom-hline.r

@@ -37,7 +37,7 @@ geom_hline <- function(mapping = NULL, data = NULL,
 #' @export
 GeomHline <- ggproto("GeomHline", Geom,
   draw_panel = function(data, panel_params, coord) {
-    ranges <- coord$range(panel_params)
+    ranges <- coord$backtransform_range(panel_params)
 
     data$x    <- ranges$x[1]
     data$xend <- ranges$x[2]

R/geom-vline.r

@@ -37,7 +37,7 @@ geom_vline <- function(mapping = NULL, data = NULL,
 #' @export
 GeomVline <- ggproto("GeomVline", Geom,
   draw_panel = function(data, panel_params, coord) {
-    ranges <- coord$range(panel_params)
+    ranges <- coord$backtransform_range(panel_params)
 
     data$x    <- data$xintercept
     data$xend <- data$xintercept

R/sf.R

@@ -543,10 +543,20 @@ CoordSf <- ggproto("CoordSf", CoordCartesian,
     )
   },
 
+  backtransform_range = function(panel_params) {
+    # this does not actually return backtransformed ranges in the general case, needs fixing
+    warning(
+      "range backtransformation not implemented in this coord; results may be wrong.",
+      call. = FALSE
+    )
+    list(x = panel_params$x_range, y = panel_params$y_range)
+  },
+
   range = function(panel_params) {
     list(x = panel_params$x_range, y = panel_params$y_range)
   },
 
+
   # CoordSf enforces a fixed aspect ratio -> axes cannot be changed freely under faceting
   is_free = function() FALSE,
 

R/summarise-plot.R

@@ -2,6 +2,45 @@
 #'
 #' These functions provide summarised information about built ggplot objects.
 #'
+#' There are three types of summary that can be obtained: A summary of the plot layout,
+#' a summary of the plot coord, and a summary of plot layers.
+#'
+#' @section Layout summary:
+#'
+#' The function `summarise_layout()` returns a table that provides information about
+#' the plot panel(s) in the built plot. The table has the following columns:
+#'
+#' \describe{
+#'   \item{`panel`}{A factor indicating the individual plot panels.}
+#'   \item{`row`}{Row number in the grid of panels.}
+#'   \item{`col`}{Column number in the grid of panels.}
+#'   \item{`vars`}{A list of lists. For each panel, the respective list
+#'     provides the variables and their values that specify the panel.}
+#'   \item{`xmin`, `xmax`}{The minimum and maximum values of the variable mapped to
+#'     the x aesthetic, in transformed coordinates.}
+#'   \item{`ymin`, `ymax`}{The minimum and maximum values of the variable mapped to
+#'     the y aesthetic, in transformed coordinates.}
+#'   \item{`xscale`}{The scale object applied to the x aesthetic.}
+#'   \item{`yscale`}{The scale object applied to the y aesthetic.}
+#' }
+#'
+#' Importantly, the values for `xmin`, `xmax`, `ymin`, `ymax`, `xscale`, and `yscale`
+#' are determined by the variables that are mapped to `x` and `y` in the `aes()` call.
+#' So even if a coord changes how x and y are shown in the final plot (as is the case
+#' for `coord_flip()` or `coord_polar()`), these changes have no effect on the results
+#' returned by `summarise_plot()`.
+#'
+#' @section Coord summary:
+#'
+#' The function `summarise_coord()` returns information about the log base for
+#' coordinates that are log-transformed in `coord_trans()`, and it also indicates
+#' whether the coord has flipped the x and y axes.
+#'
+#' @section Layer summary:
+#'
+#' The function `summarise_layers()` returns a table with a single column, `mapping`, which
+#' contains information about aesthetic mapping for each layer.
+#'
 #' @param p A ggplot_built object.
 #'
 #' @examples