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

anntzer · anntzer · commit 91f3a727620b · 2021-04-18T12:45:15.000+02:00
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° :/)
diff --git a/doc/api/next_api_changes/behavior/20012-AL.rst b/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.).
diff --git a/lib/matplotlib/projections/polar.py b/lib/matplotlib/projections/polar.py
@@ -390,6 +390,11 @@ def cla(self):
 
     def _set_scale(self, value, **kwargs):
         super()._set_scale(value, **kwargs)
+        if value == "linear":  # Should always be the case.
+            # 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):
diff --git a/lib/matplotlib/tests/test_polar.py b/lib/matplotlib/tests/test_polar.py
@@ -357,3 +357,17 @@ 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
