Backport PR matplotlib#29213: avoid-unnecessary-warning-in-_pcolorarg…

…s-function

lib/matplotlib/axes/_axes.py

@@ -5979,11 +5979,15 @@ def _pcolorargs(self, funcname, *args, shading='auto', **kwargs):
                 # grid is specified at the center, so define corners
                 # at the midpoints between the grid centers and then use the
                 # flat algorithm.
-                def _interp_grid(X):
-                    # helper for below
+                def _interp_grid(X, require_monotonicity=False):
+                    # helper for below. To ensure the cell edges are calculated
+                    # correctly, when expanding columns, the monotonicity of
+                    # X coords needs to be checked. When expanding rows, the
+                    # monotonicity of Y coords needs to be checked.
                     if np.shape(X)[1] > 1:
                         dX = np.diff(X, axis=1) * 0.5
-                        if not (np.all(dX >= 0) or np.all(dX <= 0)):
+                        if (require_monotonicity and
+                                not (np.all(dX >= 0) or np.all(dX <= 0))):
                             _api.warn_external(
                                 f"The input coordinates to {funcname} are "
                                 "interpreted as cell centers, but are not "
@@ -6003,11 +6007,11 @@ def _interp_grid(X):
                     return X
 
                 if ncols == Nx:
-                    X = _interp_grid(X)
+                    X = _interp_grid(X, require_monotonicity=True)
                     Y = _interp_grid(Y)
                 if nrows == Ny:
                     X = _interp_grid(X.T).T
-                    Y = _interp_grid(Y.T).T
+                    Y = _interp_grid(Y.T, require_monotonicity=True).T
                 shading = 'flat'
 
         C = cbook.safe_masked_invalid(C, copy=True)

lib/matplotlib/tests/test_axes.py

@@ -1589,14 +1589,37 @@ 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():