add test_stackplot in test_datetime.py (matplotlib#27114)

* add test_stackplot in test_datetime.py

* Remove stackplot test with datetime y-axis

* Refactor test_stackplot

Author: QuadroTec

lib/matplotlib/tests/test_datetime.py

@@ -325,11 +325,15 @@ def test_spy(self):
         fig, ax = plt.subplots()
         ax.spy(...)
 
-    @pytest.mark.xfail(reason="Test for stackplot not written yet")
     @mpl.style.context("default")
     def test_stackplot(self):
-        fig, ax = plt.subplots()
-        ax.stackplot(...)
+        mpl.rcParams["date.converter"] = 'concise'
+        N = 10
+        stacked_nums = np.tile(np.arange(1, N), (4, 1))
+        dates = np.array([datetime.datetime(2020 + i, 1, 1) for i in range(N - 1)])
+
+        fig, ax = plt.subplots(layout='constrained')
+        ax.stackplot(dates, stacked_nums)
 
     @pytest.mark.xfail(reason="Test for stairs not written yet")
     @mpl.style.context("default")