Draw unfilled hist()s with the zorder of lines.

anntzer · anntzer · commit abd3a25f6de1 · 2020-04-22T11:38:02.000+02:00
Unfilled hist()s (`histtype="step"`) are technically drawn as Patches
with `fill=False`, but they are effectively lines and should therefore
be drawn above gridlines (like Line2D), not below (like Patch).  Change
their default zorder accordingly.
diff --git a/doc/api/api_changes_3.3/behaviour.rst b/doc/api/api_changes_3.3/behaviour.rst
@@ -181,3 +181,8 @@ wx Timer interval
 ~~~~~~~~~~~~~~~~~
 Setting the timer interval on a not-yet-started ``TimerWx`` won't start it
 anymore.
+
+"step"-type histograms default to the zorder of `.Line2D`
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+This ensures that they go above gridlines by default.  The old ``zorder`` can
+be kept by passing it as a keyword argument to `.Axes.hist`.
diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
@@ -6769,7 +6769,8 @@ def hist(self, x, bins=None, range=None, density=False, weights=None,
                     closed=True if fill else None,
                     facecolor=c,
                     edgecolor=None if fill else c,
-                    fill=fill if fill else None))
+                    fill=fill if fill else None,
+                    zorder=None if fill else mlines.Line2D.zorder))
             for patch_list in patches:
                 for patch in patch_list:
                     if orientation == 'vertical':
diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
@@ -1903,6 +1903,18 @@ def test_hist_with_empty_input(data, expected_number_of_hists):
         assert hists.shape[0] == expected_number_of_hists
 
 
+@pytest.mark.parametrize("histtype, zorder",
+                         [("bar", mpl.patches.Patch.zorder),
+                          ("step", mpl.lines.Line2D.zorder),
+                          ("stepfilled", mpl.patches.Patch.zorder)])
+def test_hist_zorder(histtype, zorder):
+    ax = plt.figure().add_subplot()
+    ax.hist([1, 2], histtype=histtype)
+    assert ax.patches
+    for patch in ax.patches:
+        assert patch.get_zorder() == zorder
+
+
 def contour_dat():
     x = np.linspace(-3, 5, 150)
     y = np.linspace(-3, 5, 120)
