Limit helpers

NAMESPACE

@@ -149,6 +149,8 @@ export(label_scientific)
 export(label_time)
 export(label_timespan)
 export(label_wrap)
+export(limits_center)
+export(limits_include)
 export(linetype_pal)
 export(log10_trans)
 export(log1p_trans)

NEWS.md

@@ -22,6 +22,8 @@
 * Added colour manipulation functions: `col_shift()`, `col_saturate()`,
   `col_darker()`, `col_lighter()` and `col_mix()` (@teunbrand, #423)
 * `label_date_short()` gains `tz` and `locale` argument (#478)
+* New `limits_center()` and `limits_include()` functions for setting limits 
+  (#490)
 
 # scales 1.3.0
 

R/limits.R

@@ -0,0 +1,33 @@
+#' Limits include value
+#'
+#' @param value A scalar numeric value to include.
+#'
+#' @returns The `limits_()` functions return a function for setting limits.
+#' These functions take as their argument a vector values that represent the
+#' natural data limits.
+#' @export
+#'
+#' @examples
+#' # Ensure 0 is included in the scale range
+#' demo_continuous(c(5, 10), limits = limits_include(0))
+limits_include <- function(value = 0) {
+  check_number_decimal(value)
+  function(x) range(x, value)
+}
+
+#' Centered limits
+#'
+#' @param center A scalar numeric value to serve as midpoint.
+#'
+#' @returns The `limits_()` functions return a function for setting limits.
+#' These functions take as their argument a vector values that represent the
+#' natural data limits.
+#' @export
+#'
+#' @examples
+#' # Symmetry around 0
+#' demo_continuous(c(-5, 10), limits = limits_center(0))
+limits_center <- function(center = 0) {
+  check_number_decimal(center)
+  function(x) c(-1, +1) * max(abs(x - center)) + center
+}

_pkgdown.yml

@@ -46,6 +46,8 @@ reference:
   - expand_range
   - discard
   - censor
+  - limits_center
+  - limits_include
 
 - title: Transformations
   desc: >

tests/testthat/test-limits.R

@@ -0,0 +1,15 @@
+test_that("limits_include gives expected results", {
+  x <- limits_include(2)(c(-5, -1))
+  expect_equal(x, c(-5, 2))
+
+  x <- limits_include(-2)(c(5, 1))
+  expect_equal(x, c(-2, 5))
+})
+
+test_that("limits_center gives expected results", {
+  x <- limits_center(-1)(c(0, 1))
+  expect_equal(x, c(-3, 1))
+
+  x <- limits_center(1)(c(0, 1))
+  expect_equal(x, c(0, 2))
+})