Feb 15, 2024 · Feb 10, 2024
diff --git a/doc/api/next_api_changes/behavior/27767-REC.rst b/doc/api/next_api_changes/behavior/27767-REC.rst
 Legend labels for ``plot``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~

 Previously if a sequence was passed to the *label* parameter of `~.Axes.plot` when
 plotting a single dataset, the sequence was automatically cast to string for the legend
 label.  Now, if the sequence has only one element, that element will be the legend
 label.  To keep the old behavior, cast the sequence to string before passing.
diff --git a/doc/api/next_api_changes/deprecations/27767-REC.rst b/doc/api/next_api_changes/deprecations/27767-REC.rst
 Legend labels for ``plot``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~

 Previously if a sequence was passed to the *label* parameter of `~.Axes.plot` when
 plotting a single dataset, the sequence was automatically cast to string for the legend
 label.  This behavior is now deprecated and in future will error if the sequence length
 is not one (consistent with multi-dataset behavior, where the number of elements must
 match the number of datasets).  To keep the old behavior, cast the sequence to string
 before passing.
diff --git a/lib/matplotlib/axes/_base.py b/lib/matplotlib/axes/_base.py

        label = kwargs.get('label')
        n_datasets = max(ncx, ncy)
        if n_datasets > 1 and not cbook.is_scalar_or_string(label):
            if len(label) != n_datasets:
                raise ValueError(f"label must be scalar or have the same "
                                 f"length as the input data, but found "
                                 f"{len(label)} for {n_datasets} datasets.")

        if cbook.is_scalar_or_string(label):
            labels = [label] * n_datasets
        elif len(label) == n_datasets:
            labels = label
        elif n_datasets == 1:
            msg = (f'Passing label as a length {len(label)} sequence when '
                    'plotting a single dataset is deprecated in Matplotlib 3.9 '
                    'and will error in 3.11.  To keep the current behavior, '
                    'cast the sequence to string before passing.')
            _api.warn_deprecated('3.9', message=msg)
            labels = [label]
        else:
            labels = [label] * n_datasets
            raise ValueError(
                f"label must be scalar or have the same length as the input "
                f"data, but found {len(label)} for {n_datasets} datasets.")

        result = (make_artist(axes, x[:, j % ncx], y[:, j % ncy], kw,
                              {**kwargs, 'label': label})
diff --git a/lib/matplotlib/tests/test_legend.py b/lib/matplotlib/tests/test_legend.py
    x = [1, 2, 3]
    y = [2, 5, 6]
    fig, ax = plt.subplots()
    ax.plot(x, y, label=label_array)
    with pytest.warns(mpl.MatplotlibDeprecationWarning,
                      match='Passing label as a length 2 sequence'):
        ax.plot(x, y, label=label_array)
    leg = ax.legend()
    assert len(leg.get_texts()) == 1
    assert leg.get_texts()[0].get_text() == str(label_array)


 def test_plot_single_input_list_label():
    fig, ax = plt.subplots()
    line, = ax.plot([[0], [1]], label=['A'])
    assert line.get_label() == 'A'


 def test_plot_multiple_label_incorrect_length_exception():
    # check that exception is raised if multiple labels
    # are given, but number of on labels != number of lines
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,7 @@
		Legend labels for ``plot``
		~~~~~~~~~~~~~~~~~~~~~~~~~~

		Previously if a sequence was passed to the label parameter of `~.Axes.plot` when
		plotting a single dataset, the sequence was automatically cast to string for the legend
		label. Now, if the sequence has only one element, that element will be the legend
		label. To keep the old behavior, cast the sequence to string before passing.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,9 @@
		Legend labels for ``plot``
		~~~~~~~~~~~~~~~~~~~~~~~~~~

		Previously if a sequence was passed to the label parameter of `~.Axes.plot` when
		plotting a single dataset, the sequence was automatically cast to string for the legend
		label. This behavior is now deprecated and in future will error if the sequence length
		is not one (consistent with multi-dataset behavior, where the number of elements must
		match the number of datasets). To keep the old behavior, cast the sequence to string
		before passing.
Original file line number	Diff line number	Diff line change
Expand Up		@@ -513,14 +513,22 @@ def _plot_args(self, axes, tup, kwargs, *,

		label = kwargs.get('label')
		n_datasets = max(ncx, ncy)
		if n_datasets > 1 and not cbook.is_scalar_or_string(label):
		if len(label) != n_datasets:
		raise ValueError(f"label must be scalar or have the same "
		f"length as the input data, but found "
		f"{len(label)} for {n_datasets} datasets.")

		if cbook.is_scalar_or_string(label):
		labels = [label] * n_datasets
		elif len(label) == n_datasets:
		labels = label
		elif n_datasets == 1:
		msg = (f'Passing label as a length {len(label)} sequence when '
		'plotting a single dataset is deprecated in Matplotlib 3.9 '
		'and will error in 3.11. To keep the current behavior, '
		'cast the sequence to string before passing.')
		_api.warn_deprecated('3.9', message=msg)
		labels = [label]
		else:
		labels = [label] * n_datasets
		raise ValueError(
		f"label must be scalar or have the same length as the input "
		f"data, but found {len(label)} for {n_datasets} datasets.")

		result = (make_artist(axes, x[:, j % ncx], y[:, j % ncy], kw,
		{**kwargs, 'label': label})
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -1197,12 +1197,20 @@ def test_plot_single_input_multiple_label(label_array):
		x = [1, 2, 3]
		y = [2, 5, 6]
		fig, ax = plt.subplots()
		ax.plot(x, y, label=label_array)
		with pytest.warns(mpl.MatplotlibDeprecationWarning,
		match='Passing label as a length 2 sequence'):
		ax.plot(x, y, label=label_array)
		leg = ax.legend()
		assert len(leg.get_texts()) == 1
		assert leg.get_texts()[0].get_text() == str(label_array)


		def test_plot_single_input_list_label():
		fig, ax = plt.subplots()
		line, = ax.plot([[0], [1]], label=['A'])
		assert line.get_label() == 'A'


		def test_plot_multiple_label_incorrect_length_exception():
		# check that exception is raised if multiple labels
		# are given, but number of on labels != number of lines
Expand Down