matplotlib · dstansby · Feb 12, 2020 · Feb 11, 2020
diff --git a/doc/api/next_api_changes/behaviour.rst b/doc/api/next_api_changes/behaviour.rst
@@ -71,5 +71,10 @@ of *y*, the second column of *x* against the second column of *y*, **and** the
 first column of *x* against the third column of *y*.  This now raises an error
 instead.
 
-`.Text.update_from` now copies usetex state from the source Text 
+`.Text.update_from` now copies usetex state from the source Text
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+`~.Axes.stem` now defaults to ``use_line_collection=True``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+This creates the stem plot as a `.LineCollection` rather than individual
+`.Line2D` objects, greatly improving performance.
diff --git a/examples/lines_bars_and_markers/stem_plot.py b/examples/lines_bars_and_markers/stem_plot.py
@@ -12,7 +12,7 @@
 x = np.linspace(0.1, 2 * np.pi, 41)
 y = np.exp(np.sin(x))
 
-plt.stem(x, y, use_line_collection=True)
+plt.stem(x, y)
 plt.show()
 
 #############################################################################
@@ -24,7 +24,7 @@
 # control adapt the line objects returned by `~.pyplot`.
 
 markerline, stemlines, baseline = plt.stem(
-    x, y, linefmt='grey', markerfmt='D', bottom=1.1, use_line_collection=True)
+    x, y, linefmt='grey', markerfmt='D', bottom=1.1)
 markerline.set_markerfacecolor('none')
 plt.show()
 

diff --git a/examples/lines_bars_and_markers/timeline.py b/examples/lines_bars_and_markers/timeline.py
@@ -72,8 +72,7 @@
 ax.set(title="Matplotlib release dates")
 
 markerline, stemline, baseline = ax.stem(dates, levels,
-                                         linefmt="C3-", basefmt="k-",
-                                         use_line_collection=True)
+                                         linefmt="C3-", basefmt="k-")
 
 plt.setp(markerline, mec="k", mfc="w", zorder=3)
 

diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
@@ -2652,7 +2652,7 @@ def broken_barh(self, xranges, yrange, **kwargs):
 
     @_preprocess_data()
     def stem(self, *args, linefmt=None, markerfmt=None, basefmt=None, bottom=0,
-             label=None, use_line_collection=False):
+             label=None, use_line_collection=True):
         """
         Create a stem plot.
 
@@ -2709,12 +2709,12 @@ def stem(self, *args, linefmt=None, markerfmt=None, basefmt=None, bottom=0,
         label : str, default: None
             The label to use for the stems in legends.
 
-        use_line_collection : bool, default: False
+        use_line_collection : bool, default: True
             If ``True``, store and plot the stem lines as a
-            `~.collections.LineCollection` instead of individual lines. This
-            significantly increases performance, and will become the default
-            option in Matplotlib 3.3. If ``False``, defaults to the old
-            behavior of using a list of `.Line2D` objects.
+            `~.collections.LineCollection` instead of individual lines, which
+            significantly increases performance.  If ``False``, defaults to the
+            old behavior of using a list of `.Line2D` objects.  This parameter
+            may be deprecated in the future.
 
         Returns
         -------
@@ -2802,12 +2802,6 @@ def stem(self, *args, linefmt=None, markerfmt=None, basefmt=None, bottom=0,
             self.add_collection(stemlines)
         # Old behaviour is to plot each of the lines individually
         else:
-            cbook._warn_external(
-                'In Matplotlib 3.3 individual lines on a stem plot will be '
-                'added as a LineCollection instead of individual lines. '
-                'This significantly improves the performance of a stem plot. '
-                'To remove this warning and switch to the new behaviour, '
-                'set the "use_line_collection" keyword argument to True.')
             stemlines = []
             for xi, yi in zip(x, y):
                 l, = self.plot([xi, xi], [bottom, yi],

diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
@@ -2735,7 +2735,7 @@ def stackplot(
 @docstring.copy(Axes.stem)
 def stem(
         *args, linefmt=None, markerfmt=None, basefmt=None, bottom=0,
-        label=None, use_line_collection=False, data=None):
+        label=None, use_line_collection=True, data=None):
     return gca().stem(
         *args, linefmt=linefmt, markerfmt=markerfmt, basefmt=basefmt,
         bottom=bottom, label=label,

diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
@@ -3341,28 +3341,11 @@ def test_stem(use_line_collection):
                   label=' ', use_line_collection=use_line_collection)
 
     fig, ax = plt.subplots()
-    if use_line_collection:
-        ax.stem(*args, **kwargs)
-    else:
-        with pytest.warns(UserWarning):
-            ax.stem(*args, **kwargs)
+    ax.stem(*args, **kwargs)
 
     ax.legend()
 
 
-@check_figures_equal(extensions=['png'])
-def test_stem_params(fig_test, fig_ref):
-    x = np.linspace(0, 3.14, 37)
-    y = np.sin(x)
-
-    ax = fig_test.subplots()
-    ax.stem(x, y, linefmt='grey', use_line_collection=True)
-
-    ax = fig_ref.subplots()
-    with pytest.warns(UserWarning):
-        ax.stem(x, y, linefmt='grey')
-
-
 def test_stem_args():
     fig = plt.figure()
     ax = fig.add_subplot(1, 1, 1)
@@ -3371,18 +3354,18 @@ def test_stem_args():
     y = list(range(10))
 
     # Test the call signatures
-    ax.stem(y, use_line_collection=True)
-    ax.stem(x, y, use_line_collection=True)
-    ax.stem(x, y, 'r--', use_line_collection=True)
-    ax.stem(x, y, 'r--', basefmt='b--', use_line_collection=True)
+    ax.stem(y)
+    ax.stem(x, y)
+    ax.stem(x, y, 'r--')
+    ax.stem(x, y, 'r--', basefmt='b--')
 
 
 def test_stem_dates():
     fig, ax = plt.subplots(1, 1)
     xs = [dateutil.parser.parse("2013-9-28 11:00:00"),
           dateutil.parser.parse("2013-9-28 12:00:00")]
     ys = [100, 200]
-    ax.stem(xs, ys, "*-", use_line_collection=True)
+    ax.stem(xs, ys, "*-")
 
 
 @image_comparison(['hist_stacked_stepfilled_alpha'])

diff --git a/lib/matplotlib/tests/test_container.py b/lib/matplotlib/tests/test_container.py
@@ -3,7 +3,7 @@
 
 def test_stem_remove():
     ax = plt.gca()
-    st = ax.stem([1, 2], [1, 2], use_line_collection=True)
+    st = ax.stem([1, 2], [1, 2])
     st.remove()