Give each inset its own zorder band

Sibling insets all received the same _zorder (host._zorder +
INSET_ZORDER_STEP), so their figure-level artists collided inside one
band: an earlier inset's panel_border, titles, axis titles and legend
sat above a later inset's panel because they had larger numeric zorders
than the panel itself. The "later inset fully covers earlier" intent
only held for the panel area, where equal-zorder ties broke by
insertion order.

inset_element._setup now takes the 1-based index among its siblings
and assigns _zorder = parent._zorder + index * INSET_ZORDER_STEP, so
each inset occupies its own band. Insets._setup enumerates and passes
the index through.

INSET_ZORDER_STEP shrinks from 1000 to 10 now that nothing inside a
single plot reaches 99.9 anymore: watermark drops from 99.9 to 9, and
the explicit zorder=99.1 on the legend's FlexibleAnchoredOffsetbox is
removed (the mpl default of 5 is already above all decorations and
below the watermark, and ggplot._add_figure_artist offsets it into the
right band). The 0.5 gap between an inset's watermark (band top) and
the next inset's plot_background (band bottom, -0.5) is the tightest
the geometry permits and is safe — no other artist falls in it.

Author: has2k1

plotnine/composition/_inset_element.py

@@ -9,13 +9,16 @@
     from ._compose import Compose
 
 
-INSET_ZORDER_STEP = 1000
+INSET_ZORDER_STEP = 10
 """
-Zorder added to the host's value when an inset is drawn, so every
-figure-level artist on the inset (axes, plot_background, titles,
-strip text, legends, ...) sits above every figure-level artist on the
-host. Must exceed the largest existing zorder used inside a single
-plot — watermarks at 99.9 — so the host and inset stacks never overlap.
+Width of the zorder band reserved for each inset
+
+The Nth sibling inset is drawn at `host._zorder + N * INSET_ZORDER_STEP`,
+so every figure-level artist on a later inset (axes, plot_background,
+titles, strip text, legends, ...) sits above every figure-level artist
+on an earlier inset and the host. The step must exceed the largest
+within-plot figure-level zorder — watermarks at 9 — by enough that the
+next band's lowest artist (`plot_background` at -0.5) still clears it.
 """
 
 
@@ -75,12 +78,16 @@ def __post_init__(self):
                 f"bottom={self.bottom!r}, top={self.top!r}."
             )
 
-    def _setup(self, parent: ggplot):
+    def _setup(self, parent: ggplot, index: int):
         """
         Receive the host figure and zorder from parent
+
+        `index` is the 1-based position of this inset among its siblings.
+        Each sibling occupies its own zorder band so a later inset's
+        figure-level artists all sit above an earlier inset's.
         """
         self.obj.figure = parent.figure
-        self.obj._zorder = parent._zorder + INSET_ZORDER_STEP
+        self.obj._zorder = parent._zorder + index * INSET_ZORDER_STEP
 
     def draw(self):
         """
@@ -105,8 +112,8 @@ def _setup(self, parent: ggplot):
         """
         Receive the host figure and zorder for every inset
         """
-        for inset in self:
-            inset._setup(parent)
+        for i, inset in enumerate(self, start=1):
+            inset._setup(parent, i)
 
     def draw(self):
         """

plotnine/guides/guides.py

@@ -321,7 +321,6 @@ def _anchored_offset_box(boxes: list[PackerBase]):
                 bbox_to_anchor=(0, 0),
                 bbox_transform=self.plot.figure.transFigure,
                 borderpad=0.0,
-                zorder=99.1,
             )
 
         # Group together guides for each position

plotnine/watermark.py

@@ -16,12 +16,14 @@
 __all__ = ("watermark",)
 
 
-_BASE_ZORDER = 99.9
+_BASE_ZORDER = 9
 """
 Default zorder for a watermark on a top-level plot
 
 Plotnine manages the zorder of every figure-level artist so that insets
-stack predictably above their host.
+stack predictably above their host. This value must stay below
+`INSET_ZORDER_STEP - 0.5` so a sibling inset's `plot_background`
+(at `_zorder - 0.5`) clears the watermark of the inset below it.
 """