Fix for #4984

lib/matplotlib/axes/_axes.py

@@ -2824,6 +2824,14 @@ def xywhere(xs, ys, mask):
                 right = [thisx + thiserr for (thisx, thiserr)
                          in cbook.safezip(x, xerr[1])]
             else:
+                # Check if xerr is scalar or symmetric. Asymmetric is handled
+                # above. This prevents Nx2 arrays from accidentally
+                # being accepted, when the user meant the 2xN transpose.
+                if not (len(xerr) == 1 or
+                        (len(xerr) == len(x) and not (
+                            iterable(xerr[0]) and len(xerr[0]) > 1))):
+                    raise ValueError("xerr must be a scalar, the same "
+                                     "dimensions as x, or 2xN.")
                 # using list comps rather than arrays to preserve units
                 left = [thisx - thiserr for (thisx, thiserr)
                         in cbook.safezip(x, xerr)]
@@ -2882,6 +2890,12 @@ def xywhere(xs, ys, mask):
                 upper = [thisy + thiserr for (thisy, thiserr)
                          in cbook.safezip(y, yerr[1])]
             else:
+                # Check for scalar or symmetric, as in xerr.
+                if not (len(yerr) == 1 or
+                        (len(yerr) == len(y) and not (
+                            iterable(yerr[0]) and len(yerr[0]) > 1))):
+                    raise ValueError("yerr must be a scalar, the same "
+                                     "dimensions as y, or 2xN.")
                 # using list comps rather than arrays to preserve units
                 lower = [thisy - thiserr for (thisy, thiserr)
                          in cbook.safezip(y, yerr)]

lib/matplotlib/tests/test_axes.py

@@ -2038,7 +2038,8 @@ def test_errorbar():
     # Now switch to a more OO interface to exercise more features.
     fig, axs = plt.subplots(nrows=2, ncols=2, sharex=True)
     ax = axs[0, 0]
-    ax.errorbar(x, y, yerr=yerr, fmt='o')
+    # Try a Nx1 shaped error just to check
+    ax.errorbar(x, y, yerr=np.reshape(yerr, (len(y), 1)), fmt='o')
     ax.set_title('Vert. symmetric')
 
     # With 4 subplots, reduce the number of axis ticks to avoid crowding.
@@ -2064,6 +2065,21 @@ def test_errorbar():
 
     fig.suptitle('Variable errorbars')
 
+@cleanup
+def test_errorbar_shape():
+    fig = plt.figure()
+    ax = fig.gca()
+
+    x = np.arange(0.1, 4, 0.5)
+    y = np.exp(-x)
+    yerr1 = 0.1 + 0.2*np.sqrt(x)
+    yerr = np.vstack((yerr1, 2*yerr1)).T
+    xerr = 0.1 + yerr
+
+    assert_raises(ValueError, ax.errorbar, x, y, yerr=yerr, fmt='o')
+    assert_raises(ValueError, ax.errorbar, x, y, xerr=xerr, fmt='o')
+    assert_raises(ValueError, ax.errorbar, x, y, yerr=yerr, xerr=xerr, fmt='o')
+
 
 @image_comparison(baseline_images=['errorbar_limits'])
 def test_errorbar_limits():