Limits of binned scales when data has zero width

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* Binned scales with zero-width data expand the default limits by 0.1 
+  (@teunbrand, #5066)
 * New default `geom_qq_line(geom = "abline")` for better clipping in the 
   vertical direction. In addition, `slope` and `intercept` are new computed
   variables in `stat_qq_line()` (@teunbrand, #6087).

R/scale-.R

@@ -1292,9 +1292,17 @@ ScaleBinned <- ggproto("ScaleBinned", Scale,
             new_limits[1] <- breaks[1]
             breaks <- breaks[-1]
           }
-        } else {
+        } else if (nbreaks == 1) {
           bin_size <- max(breaks[1] - limits[1], limits[2] - breaks[1])
           new_limits <- c(breaks[1] - bin_size, breaks[1] + bin_size)
+        } else {
+          new_limits <- limits
+          if (zero_range(new_limits)) {
+            # 0.1 is the same width as the expansion `default_expansion()`
+            # gives for 0-width data
+            new_limits <- new_limits + c(-0.05, 0.05)
+          }
+          breaks <- new_limits
         }
         new_limits_trans <- suppressWarnings(transformation$transform(new_limits))
         limits[is.finite(new_limits_trans)] <- new_limits[is.finite(new_limits_trans)]

tests/testthat/test-scale-binned.R

@@ -104,3 +104,9 @@ test_that('binned scales can calculate breaks on date-times', {
     )))
   )
 })
+
+test_that("binned scales can calculate breaks for zero-width data", {
+  scale <- scale_x_binned()
+  scale$train(c(1, 1))
+  expect_equal(scale$get_breaks(), c(0.95, 1.05))
+})