add a test

Add a test for the monotonicity of coordinates in the test_pcolorargs function.

Author: litchi

lib/matplotlib/tests/test_axes.py

@@ -1589,14 +1589,36 @@ def test_pcolorargs():
         x = np.ma.array(x, mask=(x < 0))
     with pytest.raises(ValueError):
         ax.pcolormesh(x, y, Z[:-1, :-1])
-    # Expect a warning with non-increasing coordinates
+    # If the X or Y coords do not possess monotonicity in their respective
+    # directions, a warning indicating a bad grid will be triggered.
+    # The case of specifying coordinates by inputting 1D arrays.
     x = [359, 0, 1]
     y = [-10, 10]
     X, Y = np.meshgrid(x, y)
     Z = np.zeros(X.shape)
     with pytest.warns(UserWarning,
                       match='are not monotonically increasing or decreasing'):
         ax.pcolormesh(X, Y, Z, shading='auto')
+    # The case of specifying coordinates by inputting 2D arrays.
+    x = np.linspace(-1,1,3)
+    y = np.linspace(-1,1,3)
+    X, Y = np.meshgrid(x, y)
+    Z = np.zeros(X.shape)
+    np.random.seed(19680801)
+    noise_X = np.random.random(X.shape)
+    noise_Y = np.random.random(Y.shape)
+    with pytest.warns(UserWarning,
+        match='are not monotonically increasing or decreasing') as record:
+        # Small perturbations in coordinates will not disrupt the monotonicity
+        # of the X-coords and Y-coords in their respective directions.
+        # Therefore, no warnings will be triggered.
+        ax.pcolormesh(X+noise_X, Y+noise_Y, Z, shading='auto')
+        assert len(record) == 0
+        # Large perturbations have disrupted the monotonicity of the X-coords
+        # and Y-coords in their respective directions, thus resulting in two
+        # bad grid warnings.
+        ax.pcolormesh(X+10*noise_X, Y+10*noise_Y, Z, shading='auto')
+        assert len(record) == 2
 
 
 def test_pcolormesh_underflow_error():