Subplot title count fix + fix for issue introduced in earlier PR (#61393)

* test case for subplot stacking

* Removed overlooked print statement

* Updated test to check other subplot in figure

* Updated test cases to include more subplot stacking possibilities

* removed savefig() left in test cases

* Updated test cases to test more arrangements

* Completed function fix (order of subplot input does not matter, need clarification if it matters)

* appeasing the great pre-commit formatter

* Updated whatsnew

* Docstring adjustment

* Moved self.subplot check to a seperate bool

* Added ignore where mypy thinks self.subplots is a bool

* Actually addressed mypy typing

* Incorperated initial PR comments

* Updated missing () after .all

* Initial test cases

* Addressed more comments on PR

* Updated '&' to 'and'

* Updated Test cases

* Fixed crash when "subplots=True" is used

* Title check checks for subplot length if specified

* Updated Test cases

* Title check checks for subplot length if specified

* Updated test name

* Removed extra '_' in test name

* Fixed issue where expected_total_height returns as a df instead of a series

* Updated change notes

* Addressed mypy error

* Addresed PR comments

* Changed exception message raised and updated test case

Author: eicchen

doc/source/whatsnew/v3.0.0.rst

@@ -801,6 +801,7 @@ Plotting
 - Bug in :meth:`DataFrame.plot.bar` with ``stacked=True`` where labels on stacked bars with zero-height segments were incorrectly positioned at the base instead of the label position of the previous segment (:issue:`59429`)
 - Bug in :meth:`DataFrame.plot.line` raising ``ValueError`` when set both color and a ``dict`` style (:issue:`59461`)
 - Bug in :meth:`DataFrame.plot` that causes a shift to the right when the frequency multiplier is greater than one. (:issue:`57587`)
+- Bug in :meth:`DataFrame.plot` where ``title`` would require extra titles when plotting more than one column per subplot. (:issue:`61019`)
 - Bug in :meth:`Series.plot` preventing a line and bar from being aligned on the same plot (:issue:`61161`)
 - Bug in :meth:`Series.plot` preventing a line and scatter plot from being aligned (:issue:`61005`)
 - Bug in :meth:`Series.plot` with ``kind="pie"`` with :class:`ArrowDtype` (:issue:`59192`)

pandas/plotting/_matplotlib/core.py

@@ -802,7 +802,13 @@ def _adorn_subplots(self, fig: Figure) -> None:
         if self.title:
             if self.subplots:
                 if is_list_like(self.title):
-                    if len(self.title) != self.nseries:
+                    if not isinstance(self.subplots, bool):
+                        if len(self.subplots) != len(self.title):
+                            raise ValueError(
+                                f"The number of titles ({len(self.title)}) must equal "
+                                f"the number of subplots ({len(self.subplots)})."
+                            )
+                    elif len(self.title) != self.nseries:
                         raise ValueError(
                             "The length of `title` must equal the number "
                             "of columns if using `title` of type `list` "
@@ -1934,13 +1940,14 @@ def _make_plot(self, fig: Figure) -> None:
 
         self.subplots: list[Any]
 
-        if bool(self.subplots) and self.stacked:
-            for i, sub_plot in enumerate(self.subplots):
-                if len(sub_plot) <= 1:
-                    continue
-                for plot in sub_plot:
-                    _stacked_subplots_ind[int(plot)] = i
-                _stacked_subplots_offsets.append([0, 0])
+        if not isinstance(self.subplots, bool):
+            if bool(self.subplots) and self.stacked:
+                for i, sub_plot in enumerate(self.subplots):
+                    if len(sub_plot) <= 1:
+                        continue
+                    for plot in sub_plot:
+                        _stacked_subplots_ind[int(plot)] = i
+                    _stacked_subplots_offsets.append([0, 0])
 
         for i, (label, y) in enumerate(self._iter_data(data=data)):
             ax = self._get_ax(i)

pandas/tests/plotting/test_misc.py

@@ -31,6 +31,8 @@
 plt = pytest.importorskip("matplotlib.pyplot")
 cm = pytest.importorskip("matplotlib.cm")
 
+import re
+
 from pandas.plotting._matplotlib.style import get_standard_colors
 
 
@@ -727,7 +729,11 @@ def _df_bar_subplot_checker(df_bar_data, df_bar_df, subplot_data_df, subplot_col
         ].reset_index()
         for i in range(len(subplot_columns))
     ]
-    expected_total_height = df_bar_df.loc[:, subplot_columns].sum(axis=1)
+
+    if len(subplot_columns) == 1:
+        expected_total_height = df_bar_df.loc[:, subplot_columns[0]]
+    else:
+        expected_total_height = df_bar_df.loc[:, subplot_columns].sum(axis=1)
 
     for i in range(len(subplot_columns)):
         sliced_df = subplot_sliced_by_source[i]
@@ -743,7 +749,6 @@ def _df_bar_subplot_checker(df_bar_data, df_bar_df, subplot_data_df, subplot_col
             tm.assert_series_equal(
                 height_iter, expected_total_height, check_names=False, check_dtype=False
             )
-
         else:
             # Checks each preceding bar ends where the next one starts
             next_start_coord = subplot_sliced_by_source[i + 1]["y_coord"]
@@ -816,3 +821,44 @@ def test_bar_2_subplots_1_triple_stacked(df_bar_data, df_bar_df, subplot_divisio
         _df_bar_subplot_checker(
             df_bar_data, df_bar_df, subplot_data_df_list[i], subplot_division[i]
         )
+
+
+def test_bar_subplots_stacking_bool(df_bar_data, df_bar_df):
+    subplot_division = [("A"), ("B"), ("C"), ("D")]
+    ax = df_bar_df.plot(subplots=True, kind="bar", stacked=True)
+    subplot_data_df_list = _df_bar_xyheight_from_ax_helper(
+        df_bar_data, ax, subplot_division
+    )
+    for i in range(len(subplot_data_df_list)):
+        _df_bar_subplot_checker(
+            df_bar_data, df_bar_df, subplot_data_df_list[i], subplot_division[i]
+        )
+
+
+def test_plot_bar_label_count_default():
+    df = DataFrame(
+        [(30, 10, 10, 10), (20, 20, 20, 20), (10, 30, 30, 10)], columns=list("ABCD")
+    )
+    df.plot(subplots=True, kind="bar", title=["A", "B", "C", "D"])
+
+
+def test_plot_bar_label_count_expected_fail():
+    df = DataFrame(
+        [(30, 10, 10, 10), (20, 20, 20, 20), (10, 30, 30, 10)], columns=list("ABCD")
+    )
+    error_regex = re.escape(
+        "The number of titles (4) must equal the number of subplots (3)."
+    )
+    with pytest.raises(ValueError, match=error_regex):
+        df.plot(
+            subplots=[("A", "B")],
+            kind="bar",
+            title=["A&B", "C", "D", "Extra Title"],
+        )
+
+
+def test_plot_bar_label_count_expected_success():
+    df = DataFrame(
+        [(30, 10, 10, 10), (20, 20, 20, 20), (10, 30, 30, 10)], columns=list("ABCD")
+    )
+    df.plot(subplots=[("A", "B", "D")], kind="bar", title=["A&B&D", "C"])