BUG: disallow how keyword in shared-ax plotting (#55953)

* BUG: disallow how keyword in shared-ax plotting

* Update doc/source/whatsnew/v2.2.0.rst

Co-authored-by: Matthew Roeschke <[email protected]>

---------

Co-authored-by: Matthew Roeschke <[email protected]>

Author: 2 people

doc/source/whatsnew/v2.2.0.rst

@@ -427,6 +427,7 @@ Period
 Plotting
 ^^^^^^^^
 - Bug in :meth:`DataFrame.plot.box` with ``vert=False`` and a matplotlib ``Axes`` created with ``sharey=True`` (:issue:`54941`)
+- Bug in :meth:`Series.plot` when reusing an ``ax`` object failing to raise when a ``how`` keyword is passed (:issue:`55953`)
 -
 
 Groupby/resample/rolling

pandas/plotting/_matplotlib/timeseries.py

@@ -61,6 +61,13 @@
 
 def maybe_resample(series: Series, ax: Axes, kwargs: dict[str, Any]):
     # resample against axes freq if necessary
+
+    if "how" in kwargs:
+        raise ValueError(
+            "'how' is not a valid keyword for plotting functions. If plotting "
+            "multiple objects on shared axes, resample manually first."
+        )
+
     freq, ax_freq = _get_freq(ax, series)
 
     if freq is None:  # pragma: no cover
@@ -79,7 +86,7 @@ def maybe_resample(series: Series, ax: Axes, kwargs: dict[str, Any]):
             )
             freq = ax_freq
         elif _is_sup(freq, ax_freq):  # one is weekly
-            how = kwargs.pop("how", "last")
+            how = "last"
             series = getattr(series.resample("D"), how)().dropna()
             series = getattr(series.resample(ax_freq), how)().dropna()
             freq = ax_freq

pandas/tests/plotting/test_datetimelike.py

@@ -916,6 +916,21 @@ def test_nat_handling(self):
         assert s.index.min() <= Series(xdata).min()
         assert Series(xdata).max() <= s.index.max()
 
+    def test_to_weekly_resampling_disallow_how_kwd(self):
+        idxh = date_range("1/1/1999", periods=52, freq="W")
+        idxl = date_range("1/1/1999", periods=12, freq="ME")
+        high = Series(np.random.default_rng(2).standard_normal(len(idxh)), idxh)
+        low = Series(np.random.default_rng(2).standard_normal(len(idxl)), idxl)
+        _, ax = mpl.pyplot.subplots()
+        high.plot(ax=ax)
+
+        msg = (
+            "'how' is not a valid keyword for plotting functions. If plotting "
+            "multiple objects on shared axes, resample manually first."
+        )
+        with pytest.raises(ValueError, match=msg):
+            low.plot(ax=ax, how="foo")
+
     def test_to_weekly_resampling(self):
         idxh = date_range("1/1/1999", periods=52, freq="W")
         idxl = date_range("1/1/1999", periods=12, freq="ME")