fix: fill stairs should have lw=0 instead of edgecolor="none"

Author: andrzejnovak

doc/users/next_whats_new/modify_stairs_fill_edge_behaviour.rst

@@ -0,0 +1,9 @@
+stairs(..., fill=True) to hide patch edge by setting lw=0
+---------------------------------------------------------
+
+``stairs(..., fill=True)`` would previously hide Patch 
+edge by setting edgecolor="none". Calling ``set_color()``
+on the Patch after would make the Patch appear larger. 
+Updated treatment prevents this. Likewise calling 
+``stairs(..., fill=True, lw=3)`` will behave more 
+transparently.

lib/matplotlib/axes/_axes.py

@@ -6825,7 +6825,7 @@ def stairs(self, values, edges=None, *,
         else:
             _color = self._get_lines.get_next_color()
         if fill:
-            kwargs.setdefault('edgecolor', 'none')
+            kwargs.setdefault('linewidth', 0)
             kwargs.setdefault('facecolor', _color)
         else:
             kwargs.setdefault('edgecolor', _color)

lib/matplotlib/tests/test_axes.py

@@ -2091,7 +2091,7 @@ def test_stairs_options():
     ax.stairs(y[::-1]*3+14, x, baseline=26,
               color='purple', ls='--', lw=2, label="F")
     ax.stairs(yn[::-1]*3+15, x+1, baseline=np.linspace(27, 25, len(y)),
-              color='blue', ls='--', lw=2, label="G", fill=True)
+              color='blue', ls='--', label="G", fill=True)
     ax.stairs(y[:-1][::-1]*2+11, x[:-1]+0.5, color='black', ls='--', lw=2,
               baseline=12, hatch='//', label="H")
     ax.legend(loc=0)
@@ -2106,6 +2106,18 @@ def test_stairs_datetime():
     plt.xticks(rotation=30)
 
 
+@check_figures_equal(extensions=['png'])
+def test_stairs_edge_handling(fig_test, fig_ref):
+    # Test
+    test_ax = fig_test.add_subplot()
+    test_ax.stairs([1, 2, 3], color='red', fill=True)
+
+    # Ref
+    ref_ax = fig_ref.add_subplot()
+    st = ref_ax.stairs([1, 2, 3], fill=True)
+    st.set_color('red')
+
+
 def contour_dat():
     x = np.linspace(-3, 5, 150)
     y = np.linspace(-3, 5, 120)