Fix axvspan for drawing slices on polar plots.

There's already special-casing for polar plots in bar() (using the same
`_interpolation_steps = 100`); it seems reasonable to use the same
approach for axvspan (e.g. `polar(); axvspan(0, pi/4)`).

It seems like making axhspan work (basically drawing full rings) is less
easy, but there's an easy enough workaround
(`polar(); bar(0, bottom=1, height=1, width=2*pi)`) as the "x" (theta)
extent is actually known in that case.

Author: anntzer

doc/api/next_api_changes/behavior/20027-AL.rst

@@ -0,0 +1,3 @@
+``axvspan`` now plots full wedges in polar plots
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+... rather than triangles.

lib/matplotlib/axes/_axes.py

@@ -974,6 +974,7 @@ def axvspan(self, xmin, xmax, ymin=0, ymax=1, **kwargs):
         verts = [(xmin, ymin), (xmin, ymax), (xmax, ymax), (xmax, ymin)]
         p = mpatches.Polygon(verts, **kwargs)
         p.set_transform(self.get_xaxis_transform(which="grid"))
+        p.get_path()._interpolation_steps = 100
         self.add_patch(p)
         self._request_autoscale_view(scaley=False)
         return p

lib/matplotlib/tests/test_polar.py

@@ -373,3 +373,9 @@ def test_default_thetalocator():
         ticklocs = np.degrees(ax.xaxis.get_majorticklocs()).tolist()
         assert pytest.approx(90) in ticklocs
         assert pytest.approx(100) not in ticklocs
+
+
+def test_axvspan():
+    ax = plt.subplot(projection="polar")
+    span = plt.axvspan(0, np.pi/4)
+    assert span.get_path()._interpolation_steps > 1