add test case which hits uncovered line

Author: MarcoGorelli

pandas/plotting/_matplotlib/timeseries.py

@@ -205,6 +205,9 @@ def _use_dynamic_x(ax, data: "FrameOrSeriesUnion") -> bool:
 
     freq = get_period_alias(freq)
 
+    if freq is None:
+        return False
+
     # FIXME: hack this for 0.10.1, creating more technical debt...sigh
     if isinstance(data.index, ABCDatetimeIndex):
         base = get_freq_code(freq)[0]

pandas/tests/plotting/test_datetimelike.py

@@ -16,7 +16,7 @@
 from pandas.core.resample import DatetimeIndex
 from pandas.tests.plotting.common import TestPlotBase
 
-from pandas.tseries.offsets import DateOffset
+from pandas.tseries.offsets import DateOffset, WeekOfMonth
 
 
 @td.skip_if_no_mpl
@@ -325,6 +325,17 @@ def test_business_freq_convert(self):
         idx = ax.get_lines()[0].get_xdata()
         assert PeriodIndex(data=idx).freqstr == "M"
 
+    def test_freq_with_no_period_alias(self):
+        freq = WeekOfMonth()
+        bts = tm.makeTimeSeries(5).asfreq(freq)
+        _, ax = self.plt.subplots()
+        bts.plot(ax=ax)
+        assert ax.get_lines()[0].get_xydata()[0, 0] == bts.index[0].toordinal()
+        idx = ax.get_lines()[0].get_xdata()
+        msg = "freq not specified and cannot be inferred"
+        with pytest.raises(ValueError, match=msg):
+            PeriodIndex(data=idx)
+
     def test_nonzero_base(self):
         # GH2571
         idx = date_range("2012-12-20", periods=24, freq="H") + timedelta(minutes=30)