Backport PR #18398: Warn on non-increasing/decreasing pcolor coords

dopplershift · meeseeksmachine · commit 257a6898aaee · 2020-09-14T19:41:22.000Z
diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
@@ -5630,6 +5630,15 @@ def _interp_grid(X):
                     # helper for below
                     if np.shape(X)[1] > 1:
                         dX = np.diff(X, axis=1)/2.
+                        if not (np.all(dX >= 0) or np.all(dX <= 0)):
+                            cbook._warn_external(
+                                f"The input coordinates to {funcname} are "
+                                "not all either increasing or decreasing, "
+                                "the automatically computed edges can "
+                                "produce misleading results in this case. "
+                                "It is recommended to supply the "
+                                f"quadrilateral edges to {funcname}"
+                                f" yourself. See help({funcname}).")
                         X = np.hstack((X[:, [0]] - dX[:, [0]],
                                        X[:, :-1] + dX,
                                        X[:, [-1]] + dX[:, [-1]]))
diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
@@ -1162,6 +1162,13 @@ 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
+    x = [359, 0, 1]
+    y = [-10, 10]
+    X, Y = np.meshgrid(x, y)
+    Z = np.zeros(X.shape)
+    with pytest.warns(UserWarning, match="The input coordinates"):
+        ax.pcolormesh(X, Y, Z, shading='auto')
 
 
 @check_figures_equal(extensions=["png"])
