Hide polar spine when panel_border=element_blank()

For PolarAxes the outer circle is ax.spines['polar'], which is separate
from the rectangular panel_border patch used for Cartesian axes.
When panel_border is blank, explicitly hide the polar spine so the
theme element works consistently for polar and Cartesian plots.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: iangow

plotnine/ggplot.py

@@ -486,6 +486,11 @@ def _draw_panel_borders(self):
         # grid lines below the borders. We leave ax.patch for the
         # background only.
         if self.theme.T.is_blank("panel_border"):
+            # For PolarAxes the default circular spine is separate from the
+            # panel border Rectangle; hide it explicitly when blank.
+            for ax in self.axs:
+                if "polar" in ax.spines:
+                    ax.spines["polar"].set_visible(False)
             return
 
         from matplotlib.patches import Rectangle