BUG: Deepcopy the plot saved as last_plot

plot_context stored the in-progress plot via a shallow copy, so the
object returned by last_plot() shared mutable nested state (e.g. the
theme) with the plot being built. Building mutated that shared state,
leaving last_plot() referencing a half-built plot.

As a result, code like

    last_plot() + labs(subtitle="ab")

failed to left-align the title, because the title's alignment had
already been altered in the shared state during the previous build.

Use deepcopy so last_plot() is an independent, pristine snapshot.

Author: has2k1

plotnine/_utils/context.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from copy import deepcopy
 from dataclasses import dataclass
 from typing import TYPE_CHECKING
 
@@ -69,6 +70,7 @@ def __enter__(self) -> Self:
         Enclose in matplolib & pandas environments
         """
 
+        self._last_plot = deepcopy(self.plot)
         self.rc_context.__enter__()
         if PANDAS_LT_3:
             self.pd_option_context.__enter__()