Fix polar labels with negative theta limit

Author: dstansby

Diff for: lib/matplotlib/projections/polar.py

@@ -298,7 +298,7 @@ def set_axis(self, axis):
     def __call__(self):
         lim = self.axis.get_view_interval()
         if _is_full_circle_deg(lim[0], lim[1]):
-            return np.arange(8) * 2 * np.pi / 8
+            return np.deg2rad(min(lim)) + np.arange(8) * 2 * np.pi / 8
         else:
             return np.deg2rad(self.base())
 

Diff for: lib/matplotlib/tests/test_polar.py

@@ -446,3 +446,11 @@ def test_polar_log():
 
     n = 100
     ax.plot(np.linspace(0, 2 * np.pi, n), np.logspace(0, 2, n))
+
+
+def test_polar_neg_theta_lims():
+    fig = plt.figure()
+    ax = fig.add_subplot(projection='polar')
+    ax.set_thetalim(-np.pi, np.pi)
+    labels = [l.get_text() for l in ax.xaxis.get_ticklabels()]
+    assert labels == ['-180°', '-135°', '-90°', '-45°', '0°', '45°', '90°', '135°']