Axis alignment over multiple panels (#5826)

* Don't fix viewport size

* add null padding

* Fix viewport size, but flexibly

* add test

* add news bullet

Author: teunbrand

NEWS.md

@@ -1,5 +1,6 @@
 # ggplot2 (development version)
 
+* Axis labels are now justified across facet panels (@teunbrand, #5820)
 * Fixed bug in `stat_function()` so x-axis title now produced automatically 
   when no data added. (@phispu, #5647).
 * geom_sf now accepts shape names (@sierrajohnson, #5808)

R/guide-axis.R

@@ -424,6 +424,7 @@ GuideAxis <- ggproto(
     # Unlist the 'label' grobs
     z <- if (params$position == "left") c(2, 1, 3) else 1:3
     z <- rep(z, c(1, length(grobs$labels), 1))
+    has_labels <- !is.zero(grobs$labels[[1]])
     grobs  <- c(list(grobs$ticks), grobs$labels, list(grobs$title))
 
     # Initialise empty gtable
@@ -445,10 +446,25 @@ GuideAxis <- ggproto(
     vp <- exec(
       viewport,
       !!params$orth_aes := unit(params$orth_side, "npc"),
-      !!params$orth_size := params$measure_gtable(gt),
+      !!params$orth_size := max(params$measure_gtable(gt), unit(1, "npc")),
       just = params$opposite
     )
 
+    # Add null-unit padding to justify based on eventual gtable cell shape
+    # rather than dimensions of this axis alone.
+    if (has_labels && params$position %in% c("left", "right")) {
+      where <- layout$l[-c(1, length(layout$l))]
+      just <- with(elements$text, rotate_just(angle, hjust, vjust))$hjust %||% 0.5
+      gt <- gtable_add_cols(gt, unit(just, "null"), pos = min(where) - 1)
+      gt <- gtable_add_cols(gt, unit(1 - just, "null"), pos = max(where) + 1)
+    }
+    if (has_labels && params$position %in% c("top", "bottom")) {
+      where <- layout$t[-c(1, length(layout$t))]
+      just <- with(elements$text, rotate_just(angle, hjust, vjust))$vjust %||% 0.5
+      gt <- gtable_add_rows(gt, unit(1 - just, "null"), pos = min(where) - 1)
+      gt <- gtable_add_rows(gt, unit(just, "null"), pos = max(where) + 1)
+    }
+
     # Assemble with axis line
     absoluteGrob(
       gList(axis_line, gt),

tests/testthat/_snaps/facet-labels/outside-justified-labels.svg

@@ -157,3 +157,30 @@ test_that("parsed labels are rendered correctly", {
       facet_wrap(~ f, labeller = label_parsed)
   )
 })
+
+test_that("outside-justified labels are justified across panels", {
+
+  df <- data.frame(
+    x = c("X\nX\nX\nX\nX", "X"),
+    y = c("YYYYY", "Y"),
+    f1 = c("A", "B"),
+    f2 = c("C", "D")
+  )
+
+  # By default, axis labels are inside-justified so it doesn't matter whether
+  # justification occurs across panels. This changes for outside-justification.
+  # See #5820
+
+  p <- ggplot(df, aes(x, y)) +
+    geom_point() +
+    facet_grid(f1 ~ f2, scales = "free") +
+    guides(x.sec = "axis", y.sec = "axis") +
+    theme(
+      axis.text.y.left   = element_text(hjust = 0),
+      axis.text.y.right  = element_text(hjust = 1),
+      axis.text.x.top    = element_text(vjust = 1),
+      axis.text.x.bottom = element_text(vjust = 0)
+    )
+
+  expect_doppelganger("outside-justified labels", p)
+})