Inherit figure_size/dpi onto insets and Compose items

An inset_element's `obj` and a Compose's items share the parent's
matplotlib figure, but their own theme.figure_size and dpi default
independently. That caused the inset's `figure_size` themeable to
resize the host figure, the inset's dpi to leak into rcParams during
its draw context, and layout sites (`_plot_side_space`,
`_composition_side_space`, `margin.setup`) to read inconsistent
values.

A new `theme._inherit_figure_props` method copies these
figure-owner-only values from a parent theme. inset_element._setup
calls it on the inset's theme; Compose.draw calls it on every item
alongside the existing zorder propagation. Nested cases compose by
recursion.

Author: has2k1

plotnine/composition/_compose.py

@@ -550,11 +550,14 @@ 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.
+            # Propagate the composition's zorder & figure-owner-only
+            # theme props to its direct children before they draw, so
+            # axes are created at the right layer and child layout uses
+            # the composition's figure_size/dpi. Recursion carries the
+            # values down sub-compositions.
             for item in cmp:
                 item._zorder = cmp._zorder
+                item.theme._inherit_figure_props(cmp.theme)
 
             cmp._draw_plots()
 

plotnine/composition/_inset_element.py

@@ -47,6 +47,13 @@ class inset_element:
         - ``"plot"``  — the panel plus axes, labels, titles, captions
            and legends
         - ``"full"``  — everything the host plot occupies plus plot margin
+
+    Notes
+    -----
+    `figure_size` and `dpi` set on the inset's theme are ignored. The
+    inset shares the host's figure, so these values come from the host
+    theme. The canvas size of the inset is determined by the bounding
+    box and the area it is `align_to`.
     """
 
     obj: ggplot | Compose
@@ -88,6 +95,7 @@ def _setup(self, parent: ggplot, index: int):
         """
         self.obj.figure = parent.figure
         self.obj._zorder = parent._zorder + index * INSET_ZORDER_STEP
+        self.obj.theme._inherit_figure_props(parent.theme)
 
     def draw(self):
         """

plotnine/themes/theme.py

@@ -472,6 +472,20 @@ def to_retina(self) -> theme:
         self._is_retina = True
         return self
 
+    def _inherit_figure_props(self, other: theme) -> None:
+        """
+        Copy themeables that modify the figure
+
+        Used when this theme is attached to a plot that does not own
+        its figure (an inset, or a member of a composition). Such a plot
+        has no figure to size or DPI; the values must come from the
+        figure's owner.
+        """
+        self += theme(
+            figure_size=other.getp("figure_size"),
+            dpi=other.getp("dpi"),
+        )
+
     def _smart_title_and_subtitle_ha(
         self, title: str | None, subtitle: str | None
     ):