MAINT: Warn on non-increasing/decreasing pcolor coords

greglucas · greglucas · commit 75cb8104b384 · 2020-09-02T15:55:13.000-06:00
pcolor* will now automatically calculate bin edges for users
if shading is 'auto' or 'nearest'. This can lead to surprising
resulting edge calculations if the coordinate arrays are not
sorted prior to calling the function.
diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
@@ -5682,6 +5682,15 @@ 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
@@ -1131,6 +1131,13 @@ 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"])
