Fix matplotlib#28016: wrong lower ylim when baseline=None on stairs.

Instead of assuming that lower ylim=0 when baseline=None it now
takes into account the imputed values. Also no longer uses sticky_edges
in these cases. Changed the unit test that tested baseline=None.

Author: DiogoCasteloCardoso

lib/matplotlib/axes/_axes.py

@@ -7272,14 +7272,14 @@ def stairs(self, values, edges=None, *,
                 "very likely that the resulting fill patterns is not the desired "
                 "result."
             )
-        if baseline is None:
-            baseline = 0
-        if orientation == 'vertical':
-            patch.sticky_edges.y.append(np.min(baseline))
-            self.update_datalim([(edges[0], np.min(baseline))])
-        else:
-            patch.sticky_edges.x.append(np.min(baseline))
-            self.update_datalim([(np.min(baseline), edges[0])])
+
+        if baseline is not None:
+            if orientation == 'vertical':
+                patch.sticky_edges.y.append(np.min(baseline))
+                self.update_datalim([(edges[0], np.min(baseline))])
+            else:
+                patch.sticky_edges.x.append(np.min(baseline))
+                self.update_datalim([(np.min(baseline), edges[0])])
         self._request_autoscale_view()
         return patch
 

lib/matplotlib/tests/test_axes.py

@@ -2449,16 +2449,17 @@ def test_stairs_update(fig_test, fig_ref):
 
 
 @check_figures_equal(extensions=['png'])
-def test_stairs_baseline_0(fig_test, fig_ref):
-    # Test
-    test_ax = fig_test.add_subplot()
-    test_ax.stairs([5, 6, 7], baseline=None)
+def test_stairs_baseline_None(fig_test, fig_ref):
+    x = np.array([0, 2, 3, 5, 10])
+    y = np.array([1.148, 1.231, 1.248, 1.25])
+
+    test_axes = fig_test.add_subplot()
+    test_axes.stairs(y, x, baseline=None)
 
-    # Ref
-    ref_ax = fig_ref.add_subplot()
     style = {'solid_joinstyle': 'miter', 'solid_capstyle': 'butt'}
-    ref_ax.plot(range(4), [5, 6, 7, 7], drawstyle='steps-post', **style)
-    ref_ax.set_ylim(0, None)
+
+    ref_axes = fig_ref.add_subplot()
+    ref_axes.plot(x, np.append(y, y[-1]), drawstyle='steps-post', **style)
 
 
 def test_stairs_empty():