fix guide_axis_theta: use 'auto' mode so angle is relative to tangent

In Matplotlib, ThetaTick has two label-rotation modes:
  ('default', N) — absolute N degrees (what tick_params(labelrotation=N) sets)
  ('auto', N)    — tangential direction + N degrees offset

ggplot2's guide_axis_theta(angle=0) means labels tangential to the arc
(0° offset from the tangent), not horizontal. Patching tick._labelrotation
directly is the only way to set 'auto' mode with a user-angle offset since
ax.tick_params() always sets 'default' mode.

Author: iangow

plotnine/coords/coord_radial.py

@@ -335,10 +335,13 @@ def setup_ax(
         if self.theta_labels or self.end is not None:
             ax.tick_params(axis="x", pad=self.theta_label_pad)
             if (angle := self._theta_guide_angle(theme)) is not None:
-                ax.tick_params(axis="x", labelrotation=angle)
-                for label in ax.get_xticklabels():
-                    label.set_rotation(angle)
-                    label.set_rotation_mode("anchor")
+                # Use Matplotlib's 'auto' mode so labels orient tangentially
+                # to the arc, with `angle` as an offset — matching ggplot2's
+                # guide_axis_theta() semantics where angle=0 means tangential.
+                # ax.tick_params(labelrotation=...) always sets 'default' mode
+                # (absolute degrees), so we patch each tick directly instead.
+                for tick in ax.xaxis.get_major_ticks():
+                    tick._labelrotation = ("auto", angle)
         # Allow geom_text labels to extend past the polar axes bounding box
         # (e.g. spoke labels placed just beyond the outermost bar tip).
         for text in ax.texts: