fix(layout): reserve axis text margins in facet gullies

The free-scale gully terms added the tick and label extents but not
the label margins, though the drawn labels honor them (the margin is
applied as the tick pad). With a large margin and small panel_spacing
the labels of one panel overlapped the neighbouring panel.

Author: has2k1

doc/changelog.qmd

@@ -108,6 +108,10 @@ title: Changelog
 
 ### Bug Fixes
 
+- The space between facet panels now accounts for the margins of the axis
+  text, so with free scales large margins no longer push the tick labels
+  into the neighbouring panel.
+
 - Subclass geoms now inherit their parent geom's default parameters, so
   parameters that a parent geom supports are no longer rejected.
   [](:class:`~plotnine.geom_step`) accepts `lineend`, `linejoin` and `arrow`,

plotnine/_mpl/layout_manager/_plot_side_space.py

@@ -1241,16 +1241,26 @@ def _calculate_panel_spacing_facet_wrap(self) -> tuple[float, float]:
         else:
             self.sw += space.strip_text + space.strip_switch_pad
 
+        # Per-panel axes claim their ticks, labels and label margins in
+        # the gullies.
         if facet.free["x"]:
             for side in ("bottom", "top"):
-                self.sh += self.items.axis_text_x_max_height_at(
+                text = self.items.axis_text_x_max_height_at("all", side)
+                if text:
+                    m = theme.get_margin(f"axis_text_x_{side}").fig
+                    text += m.t + m.b
+                self.sh += text + self.items.axis_ticks_x_max_height_at(
                     "all", side
-                ) + self.items.axis_ticks_x_max_height_at("all", side)
+                )
         if facet.free["y"]:
             for side in ("left", "right"):
-                self.sw += self.items.axis_text_y_max_width_at(
+                text = self.items.axis_text_y_max_width_at("all", side)
+                if text:
+                    m = theme.get_margin(f"axis_text_y_{side}").fig
+                    text += m.l + m.r
+                self.sw += text + self.items.axis_ticks_y_max_width_at(
                     "all", side
-                ) + self.items.axis_ticks_y_max_width_at("all", side)
+                )
 
         # width and height of axes as fraction of figure width & height
         self.w = (self.panel_width - self.sw * (ncol - 1)) / ncol

tests/test_layout.py

@@ -115,6 +115,21 @@ def test_facet_wrap_scales_free(self):
         p = self.g + facet_wrap("carb", scales="free")
         assert p == "facet_wrap_scales_free"
 
+    def test_facet_wrap_scales_free_text_margin(self):
+        # The gullies fit the axis text margins; with zero panel
+        # spacing the labels touch the neighbouring panel but do not
+        # overlap it.
+        p = (
+            self.g
+            + facet_wrap("carb", scales="free")
+            + theme(
+                panel_spacing=0,
+                axis_text_x=element_text(margin={"t": 5}),
+                axis_text_y=element_text(margin={"r": 5}),
+            )
+        )
+        assert p == "facet_wrap_scales_free_text_margin"
+
     def test_plot_margin_aspect_ratio(self):
         # The margin should be exact in both directions even if
         # the figure has an aspect ratio != 1.