TST: Debug flaky plotting test (pandas-dev#19925)

pandas/tests/plotting/test_datetimelike.py

@@ -689,14 +689,17 @@ def test_mixed_freq_regular_first(self):
         s2 = s1[[0, 5, 10, 11, 12, 13, 14, 15]]
 
         # it works!
-        s1.plot()
+        _, ax = self.plt.subplots()
+        s1.plot(ax=ax)
 
-        ax2 = s2.plot(style='g')
+        ax2 = s2.plot(style='g', ax=ax)
         lines = ax2.get_lines()
         idx1 = PeriodIndex(lines[0].get_xdata())
         idx2 = PeriodIndex(lines[1].get_xdata())
-        assert idx1.equals(s1.index.to_period('B'))
-        assert idx2.equals(s2.index.to_period('B'))
+
+        tm.assert_index_equal(idx1, s1.index.to_period('B'))
+        tm.assert_index_equal(idx2, s2.index.to_period('B'))
+
         left, right = ax2.get_xlim()
         pidx = s1.index.to_period()
         assert left <= pidx[0].ordinal