If stem markerfmt does not contain a color, use the color from linefmt

instead of the hard-coded 'C0'. While this is technically a breaking
API change, it is unlikely that somebody wants to color their lines,
but intentionally leaves their marker color as default 'C0'. I think
there is no simple mechanism that'd allow a deprecation. All I can
think of would require user actions at the beginning and at the end
of the deprecation period. Given that this should only affect very
few users and there is a simple workaround (state both formats
explicitly).

Author: timhoffm

doc/api/next_api_changes/behavior/23233-TH.rst

@@ -0,0 +1,13 @@
+``stem(..., markerfmt=...)`` behavior
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The behavior of the *markerfmt* parameter of `~.Axes.stem` has changed:
+
+- If *markerfmt* does not contain a color, the color is taken from *linefmt*.
+- If *markerfmt* does not contain a marker, the default is 'o'.
+
+Before, *markerfmt* was passed unmodified to ``plot(..., fmt)``, which had
+a number of unintended side-effects; e.g. only giving a color switched to
+a solid line without markers.
+
+For a simple call ``stem(x, y)`` without parameters, the new rules still
+reproduce the old behavior.

lib/matplotlib/axes/_axes.py

@@ -2821,6 +2821,8 @@ def stem(self, *args, linefmt=None, markerfmt=None, basefmt=None, bottom=0,
 
         The *locs*-positions are optional. The formats may be provided either
         as positional or as keyword-arguments.
+        Passing *markerfmt* and *basefmt* positionally is deprecated since
+        Matplotlib 3.5.
 
         Parameters
         ----------
@@ -2853,8 +2855,8 @@ def stem(self, *args, linefmt=None, markerfmt=None, basefmt=None, bottom=0,
 
         markerfmt : str, optional
             A string defining the color and/or shape of the markers at the stem
-            heads.  Default: 'C0o', i.e. filled circles with the first color of
-            the color cycle.
+            heads. If the marker is not given, use the marker 'o', i.e. filled
+            circles. If the color is not given, use the color from *linefmt*.
 
         basefmt : str, default: 'C3-' ('C2-' in classic mode)
             A format string defining the properties of the baseline.
@@ -2918,16 +2920,27 @@ def stem(self, *args, linefmt=None, markerfmt=None, basefmt=None, bottom=0,
         else:  # horizontal
             heads, locs = self._process_unit_info([("x", heads), ("y", locs)])
 
-        # defaults for formats
+        # resolve line format
         if linefmt is None:
             linefmt = args[0] if len(args) > 0 else "C0-"
         linestyle, linemarker, linecolor = _process_plot_format(linefmt)
 
+        # resolve marker format
         if markerfmt is None:
-            markerfmt = args[1] if len(args) > 1 else "C0o"
+            # if not given as kwarg, check for positional or fall back to 'o'
+            markerfmt = args[1] if len(args) > 1 else "o"
+        if markerfmt == '':
+            markerfmt = ' '  # = empty line style; '' would resolve rcParams
         markerstyle, markermarker, markercolor = \
             _process_plot_format(markerfmt)
-
+        if markermarker is None:
+            markermarker = 'o'
+        if markerstyle is None:
+            markerstyle = 'None'
+        if markercolor is None:
+            markercolor = linecolor
+
+        # resolve baseline format
         if basefmt is None:
             basefmt = (args[2] if len(args) > 2 else
                        "C2-" if rcParams["_internal.classic_mode"] else "C3-")

lib/matplotlib/tests/test_axes.py

@@ -3833,6 +3833,7 @@ def test_stem(use_line_collection):
 
 
 def test_stem_args():
+    """Test that stem() correctly identifies x and y values."""
     def _assert_equal(stem_container, expected):
         x, y = map(list, stem_container.markerline.get_data())
         assert x == expected[0]
@@ -3853,6 +3854,71 @@ def _assert_equal(stem_container, expected):
     _assert_equal(ax.stem(y, 'r--'), expected=([0, 1, 2], y))
 
 
+def test_stem_markerfmt():
+    """Test that stem(..., markerfmt=...) produces the intended markers."""
+    def _assert_equal(stem_container, linecolor=None, markercolor=None,
+                      marker=None):
+        """
+        Check that the given StemContainer has the properties listed as
+        keyword-arguments.
+        """
+        if linecolor is not None:
+            assert mcolors.same_color(
+                stem_container.stemlines.get_color(),
+                linecolor)
+        if markercolor is not None:
+            assert mcolors.same_color(
+                stem_container.markerline.get_color(),
+                markercolor)
+        if marker is not None:
+            assert stem_container.markerline.get_marker() == marker
+        assert stem_container.markerline.get_linestyle() == 'None'
+
+    fig, ax = plt.subplots()
+
+    x = [1, 3, 5]
+    y = [9, 8, 7]
+
+    # no linefmt
+    _assert_equal(ax.stem(x, y), markercolor='C0', marker='o')
+    _assert_equal(ax.stem(x, y, markerfmt='x'), markercolor='C0', marker='x')
+    _assert_equal(ax.stem(x, y, markerfmt='rx'), markercolor='r', marker='x')
+
+    # positional linefmt
+    _assert_equal(
+        ax.stem(x, y, 'r'),  # marker color follows linefmt if not given
+        linecolor='r', markercolor='r', marker='o')
+    _assert_equal(
+        ax.stem(x, y, 'rx'),  # the marker is currently not taken from linefmt
+        linecolor='r', markercolor='r', marker='o')
+    _assert_equal(
+        ax.stem(x, y, 'r', markerfmt='x'),  # only marker type specified
+        linecolor='r', markercolor='r', marker='x')
+    _assert_equal(
+        ax.stem(x, y, 'r', markerfmt='g'),  # only marker color specified
+        linecolor='r', markercolor='g', marker='o')
+    _assert_equal(
+        ax.stem(x, y, 'r', markerfmt='gx'),  # marker type and color specified
+        linecolor='r', markercolor='g', marker='x')
+    _assert_equal(
+        ax.stem(x, y, 'r', markerfmt=' '),  # markerfmt=' ' for no marker
+        linecolor='r', markercolor='r', marker='None')
+    _assert_equal(
+        ax.stem(x, y, 'r', markerfmt=''),  # markerfmt='' for no marker
+        linecolor='r', markercolor='r', marker='None')
+
+    # with linefmt kwarg
+    _assert_equal(
+        ax.stem(x, y, linefmt='r'),
+        linecolor='r', markercolor='r', marker='o')
+    _assert_equal(
+        ax.stem(x, y, linefmt='r', markerfmt='x'),
+        linecolor='r', markercolor='r', marker='x')
+    _assert_equal(
+        ax.stem(x, y, linefmt='r', markerfmt='gx'),
+        linecolor='r', markercolor='g', marker='x')
+
+
 def test_stem_dates():
     fig, ax = plt.subplots(1, 1)
     xs = [dateutil.parser.parse("2013-9-28 11:00:00"),