Fix default theta tick locations for non-full-circle polar plots.

Currently, non-full-circle polar plots may (depending on the axes size)
put theta ticks at positions that make sense for linear plots, but are a
bit weird for polar plots (e.g. 50° or 25°).  Instead, prefer "standard"
angles (90°, 45°, 30°, 15°, 10°).  For really small axes, there's still
a problem because a single tick may end up at 150°, but fixing the
machinery to allow 15° while rejecting 150° would be trickier.
(Likewise, really thin slivers likely want ticks at 0.5° steps rather
than 0.45° :/)

Author: anntzer

doc/api/next_api_changes/behavior/20012-AL.rst

@@ -0,0 +1,5 @@
+Default theta tick locations for non-full-circle polar plots have changed
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+For polar plots that don't cover a full circle, the default theta tick
+locations are now at multiples of 10°, 15°, 30°, 45°, 90°, rather than using
+values that mostly make sense for linear plots (20°, 25°, etc.).

lib/matplotlib/projections/polar.py

@@ -393,6 +393,10 @@ def _set_scale(self, value, **kwargs):
             raise NotImplementedError(
                 "The xscale cannot be set on a polar plot")
         super()._set_scale(value, **kwargs)
+        # LinearScale.set_default_locators_and_formatters just set the major
+        # locator to be an AutoLocator, so we customize it here to have ticks
+        # at sensible degree multiples.
+        self.get_major_locator().set_params(steps=[1, 1.5, 3, 4.5, 9, 10])
         self._wrap_locator_formatter()
 
     def _copy_tick_props(self, src, dest):

lib/matplotlib/tests/test_polar.py

@@ -249,6 +249,8 @@ def test_polar_theta_limits():
                            direction=DIRECTIONS[i % len(DIRECTIONS)],
                            rotation='auto')
             ax.yaxis.set_tick_params(label2On=True, rotation='auto')
+            ax.xaxis.get_major_locator().base.set_params(  # backcompat
+                steps=[1, 2, 2.5, 5, 10])
 
 
 @check_figures_equal(extensions=["png"])
@@ -357,3 +359,17 @@ def test_thetalim_args():
     assert tuple(np.radians((ax.get_thetamin(), ax.get_thetamax()))) == (0, 1)
     ax.set_thetalim((2, 3))
     assert tuple(np.radians((ax.get_thetamin(), ax.get_thetamax()))) == (2, 3)
+
+
+def test_default_thetalocator():
+    # Ideally we would check AAAABBC, but the smallest axes currently puts a
+    # single tick at 150° because MaxNLocator doesn't have a way to accept 15°
+    # while rejecting 150°.
+    fig, axs = plt.subplot_mosaic(
+        "AAAABB.", subplot_kw={"projection": "polar"})
+    for ax in axs.values():
+        ax.set_thetalim(0, np.pi)
+    for ax in axs.values():
+        ticklocs = np.degrees(ax.xaxis.get_majorticklocs()).tolist()
+        assert pytest.approx(90) in ticklocs
+        assert pytest.approx(100) not in ticklocs