Background wraps the user bbox for image insets

The background rectangle on an image inset now tracks the full user
bbox (the letterbox envelope) instead of the fitted image. A themed
fill or border now surrounds the whole requested region, with the
fill visible in the letterbox padding bands.

The patch is now added before the image artist so its fill renders
behind the image; previously it sat on top because it hugged the
image and only the border was meant to be visible.

Author: has2k1

plotnine/composition/_inset_image.py

@@ -95,8 +95,10 @@ def _arrange_in_box(
 
         The image is letterboxed inside the box with transparent
         padding on the two opposing edges, positioned by the
-        configured `anchor`. The background rectangle tracks the
-        same fitted box.
+        configured `anchor`. The background rectangle wraps the full
+        user bbox (the letterbox envelope), so a themed fill or
+        border surrounds the entire requested region — not just the
+        fitted image.
 
         Parameters
         ----------
@@ -113,22 +115,24 @@ def _arrange_in_box(
             self.figure,
             anchor=self._anchor,
         )
-        w, h = r - l, t - b
-        self._frac_bbox.bounds = (l, b, w, h)  # pyright: ignore[reportAttributeAccessIssue]
-        self.patch.set_bounds(l, b, w, h)
+        self._frac_bbox.bounds = (l, b, r - l, t - b)  # pyright: ignore[reportAttributeAccessIssue]
+        self.patch.set_bounds(left, bottom, right - left, top - bottom)
 
     def draw(self):
         from matplotlib.image import BboxImage
         from matplotlib.transforms import TransformedBbox
 
+        # Background first so its fill sits behind the image — the
+        # letterbox.
+        self.theme._setup(self)  # pyright: ignore[reportArgumentType]
+        self._draw_plot_background()
+
         image_artist = BboxImage(
             TransformedBbox(self._frac_bbox, self.figure.transFigure)
         )
         image_artist.set_data(np.asarray(self._image))
         self.figure.add_artist(image_artist)
 
-        self.theme._setup(self)  # pyright: ignore[reportArgumentType]
-        self._draw_plot_background()
         self.theme.apply()
 
     def _draw_plot_background(self):