Add scatter test for datetime units (matplotlib#26882)

* test_axes

added 3 subplots with datetime array on the x axis, y axis, and both axes.

* incorporated issues in previous PR

* deleted whitespaces after end of line

* got rid of space in 311

* Simplified datetime array

* fixed mistake in previous commit

* Update test_datetime.py

I needed to rotate axis labels because they were still clogged after I reduced array length

* Update test_datetime.py

eliminated unused variable according to feedback ('lims')

Author: seohyeonlee2020

lib/matplotlib/tests/test_datetime.py

@@ -301,11 +301,27 @@ def test_quiverkey(self):
         fig, ax = plt.subplots()
         ax.quiverkey(...)
 
-    @pytest.mark.xfail(reason="Test for scatter not written yet")
     @mpl.style.context("default")
     def test_scatter(self):
-        fig, ax = plt.subplots()
-        ax.scatter(...)
+        mpl.rcParams["date.converter"] = 'concise'
+        base = datetime.datetime(2005, 2, 1)
+        dates = [base + datetime.timedelta(hours=(2 * i)) for i in range(10)]
+        N = len(dates)
+        np.random.seed(19680801)
+        y = np.cumsum(np.random.randn(N))
+        fig, axs = plt.subplots(3, 1, layout='constrained', figsize=(6, 6))
+        # datetime array on x axis
+        axs[0].scatter(dates, y)
+        for label in axs[0].get_xticklabels():
+            label.set_rotation(40)
+            label.set_horizontalalignment('right')
+        # datetime on y axis
+        axs[1].scatter(y, dates)
+        # datetime on both x, y axes
+        axs[2].scatter(dates, dates)
+        for label in axs[2].get_xticklabels():
+            label.set_rotation(40)
+            label.set_horizontalalignment('right')
 
     @pytest.mark.xfail(reason="Test for semilogx not written yet")
     @mpl.style.context("default")