FIX: pcolor writing to read-only input mask

Author: Yi Wei

lib/matplotlib/axes/_axes.py

@@ -5773,7 +5773,7 @@ def _pcolorargs(self, funcname, *args, shading='auto', **kwargs):
             # unit conversion allows e.g. datetime objects as axis values
             X, Y = args[:2]
             X, Y = self._process_unit_info([("x", X), ("y", Y)], kwargs)
-            X, Y = [cbook.safe_masked_invalid(a) for a in [X, Y]]
+            X, Y = [cbook.safe_masked_invalid(a, copy=True) for a in [X, Y]]
 
             if funcname == 'pcolormesh':
                 if np.ma.is_masked(X) or np.ma.is_masked(Y):

lib/matplotlib/tests/test_axes.py

@@ -1507,11 +1507,17 @@ def test_pcolorargs_with_read_only():
     y = np.linspace(0, 1, 10)
     X, Y = np.meshgrid(x, y)
     Z = np.sin(2 * np.pi * X) * np.cos(2 * np.pi * Y)
-    Zmask = np.broadcast_to([True, False] * 5, Z.shape)
-    assert Zmask.flags.writeable is False
-    masked_Z = np.ma.array(Z, mask=Zmask)
+    mask = np.broadcast_to([True, False] * 5, Z.shape)
+    assert mask.flags.writeable is False
+    masked_Z = np.ma.array(Z, mask=mask)
     plt.pcolormesh(X, Y, masked_Z)
 
+    masked_X = np.ma.array(X, mask=mask)
+    masked_Y = np.ma.array(Y, mask=mask)
+    with pytest.warns(UserWarning,
+                      match='are not monotonically increasing or decreasing'):
+        plt.pcolor(masked_X, masked_Y, masked_Z)
+
 
 @check_figures_equal(extensions=["png"])
 def test_pcolornearest(fig_test, fig_ref):