Set axes zorder at creation

Pass the owning plot's _zorder to add_subplot in facet._make_axes so
every panel axes is born at the right layer. Compose.draw propagates
the composition's _zorder to its direct children before drawing, and
the recursion carries the value down sub-compositions. Combined with
ggplot._draw_insets raising _zorder by 1 per inset boundary, insets
(including Compose insets and nested insets) end up above the host
without any post-hoc subtree walk.

Author: has2k1

plotnine/composition/_compose.py

@@ -549,6 +549,13 @@ def draw(self, *, show: bool = False) -> Figure:
 
         def _draw(cmp):
             figure = cmp._setup()
+
+            # Propagate the composition's zorder to its direct children
+            # before they draw, so axes are created at the right layer.
+            # Recursion carries the value down sub-compositions.
+            for item in cmp:
+                item._zorder = cmp._zorder
+
             cmp._draw_plots()
 
             for sub_cmp in cmp.iter_sub_compositions():

plotnine/facets/facet.py

@@ -399,7 +399,9 @@ def _make_axes(self) -> tuple[p9GridSpec, list[Axes]]:
         # Create axes
         it = itertools.product(range(self.nrow), range(self.ncol))
         for i, (row, col) in enumerate(it):
-            axsarr[row, col] = self.figure.add_subplot(gs[i])
+            axsarr[row, col] = self.figure.add_subplot(
+                gs[i], zorder=self.plot._zorder
+            )
 
         # Rearrange axes
         # They are ordered to match the positions in the layout table