matplotlib · afvincent · Jun 1, 2018 · Jun 1, 2018 · Jun 1, 2018 · Jun 1, 2018
diff --git a/doc/api/next_api_changes/2018-06-01-AFV.rst b/doc/api/next_api_changes/2018-06-01-AFV.rst
@@ -0,0 +1,32 @@
+Change of the (default) legend handler for Line2D instances
+-----------------------------------------------------------
+
+The default legend handler for Line2D instances (`.HandlerLine2D`) now
+consistently exposes all the attributes and methods related to the line
+marker (:ghissue:`11358`). This makes easy to change the marker features
+after instantiating a legend.
+
+.. code::
+
+  import matplotlib.pyplot as plt
+
+  fig, ax = plt.subplots()
+
+  ax.plot([1, 3, 2], marker="s", label="Line", color="pink", mec="red", ms=8)
+  leg = ax.legend()
+
+  leg.legendHandles[0].set_color("lightgray")
+  leg.legendHandles[0].set_mec("black")  # marker edge color
+
+The former legend handler for Line2D objects has been renamed
+`.HandlerLine2DCompound`. To revert to the behavior that was used before
+Matplotlib 3, one can use
+
+.. code::
+
+  import matplotlib.legend as mlegend
+  from matplotlib.legend_handler import HandlerLine2DCompound
+  from matplotlib.lines import Line2D
+
+  mlegend.Legend.update_default_handler_map({Line2D: HandlerLine2DCompound()})
+
diff --git a/lib/matplotlib/legend_handler.py b/lib/matplotlib/legend_handler.py
@@ -203,9 +203,10 @@ def get_ydata(self, legend, xdescent, ydescent, width, height, fontsize):
         return ydata
 
 
-class HandlerLine2D(HandlerNpoints):
+class HandlerLine2DCompound(HandlerNpoints):
     """
-    Handler for `.Line2D` instances.
+    Original handler for `.Line2D` instances, that relies on combining
+    a line-only with a marker-only artist.
     """
     def __init__(self, marker_pad=0.3, numpoints=None, **kw):
         """
@@ -255,6 +256,62 @@ def create_artists(self, legend, orig_handle,
         return [legline, legline_marker]
 
 
+class HandlerLine2D(HandlerNpoints):
+    """
+    Handler for `.Line2D` instances.
+
+    A class similar to the original handler for `.Line2D` instances but
+    that uses a monolithic artist rather than using one artist for the
+    line and another one for the marker(s).  NB: the former handler, in
+    use before Matplotlib 3, is still available as `.HandlerLine2DCompound`.
+
+    """
+    def __init__(self, marker_pad=0.3, numpoints=None, **kw):
+        """
+        Parameters
+        ----------
+        marker_pad : float
+            Padding between points in legend entry.
+
+        numpoints : int
+            Number of points to show in legend entry.
+
+        Notes
+        -----
+        Any other keyword arguments are given to `HandlerNpoints`.
+        """
+        HandlerNpoints.__init__(self, marker_pad=marker_pad,
+                                numpoints=numpoints, **kw)
+
+    def create_artists(self, legend, orig_handle,
+                       xdescent, ydescent, width, height, fontsize,
+                       trans):
+
+        xdata, xdata_marker = self.get_xdata(legend, xdescent, ydescent,
+                                             width, height, fontsize)
+
+        markevery = None
+        if self.get_numpoints(legend) == 1:
+            # Special case: one wants a single marker in the center
+            # and a line that extends on both sides. One will use a
+            # 3 points line, but only mark the #1 (i.e. middle) point.
+            xdata = np.linspace(xdata[0], xdata[-1], 3)
+            markevery = [1]
+
+        ydata = ((height - ydescent) / 2.) * np.ones(xdata.shape, float)
+        legline = Line2D(xdata, ydata, markevery=markevery)
+
+        self.update_prop(legline, orig_handle, legend)
+
+        if legend.markerscale != 1:
+            newsz = legline.get_markersize() * legend.markerscale
+            legline.set_markersize(newsz)
+
+        legline.set_transform(trans)
+
+        return [legline]
+
+
 class HandlerPatch(HandlerBase):
     """
     Handler for `.Patch` instances.

diff --git a/lib/matplotlib/lines.py b/lib/matplotlib/lines.py
@@ -801,8 +801,13 @@ def draw(self, renderer):
                 # subsample the markers if markevery is not None
                 markevery = self.get_markevery()
                 if markevery is not None:
+                    try:
+                        transform = self.axes.transAxes
+                    except AttributeError:
+                        # Typically in the case of a **figure** legend.
+                        transform = self.get_transform()
                     subsampled = _mark_every_path(markevery, tpath,
-                                                  affine, self.axes.transAxes)
+                                                  affine, transform)
                 else:
                     subsampled = tpath
 

diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
@@ -1329,9 +1329,13 @@ def test_markevery():
     ax.legend()
 
 
-@image_comparison(baseline_images=['markevery_line'],
+@image_comparison(baseline_images=['markevery_line'], tol=0.005,
                   remove_text=True)
 def test_markevery_line():
+    # TODO: a slight change in rendering between Inkscape versions may explain
+    # why one had to introduce a small non-zero tolerance for the SVG test
+    # to pass. One may try to remove this hack once Travis' Inkscape version
+    # is modern enough. FWIW, no failure with 0.92.3 on my computer (#11358).
     x = np.linspace(0, 10, 100)
     y = np.sin(x) * np.sqrt(x/10 + 0.5)
 

diff --git a/lib/matplotlib/tests/test_legend.py b/lib/matplotlib/tests/test_legend.py
@@ -10,6 +10,7 @@
 import matplotlib as mpl
 import matplotlib.transforms as mtransforms
 import matplotlib.collections as mcollections
+import matplotlib.lines as mlines
 from matplotlib.legend_handler import HandlerTuple
 import matplotlib.legend as mlegend
 from matplotlib.cbook.deprecation import MatplotlibDeprecationWarning
@@ -55,9 +56,13 @@ def test_legend_auto2():
     ax.legend([b1[0], b2[0]], ['up', 'down'], loc='best')
 
 
-@image_comparison(baseline_images=['legend_auto3'])
+@image_comparison(baseline_images=['legend_auto3'], tol=0.002)
 def test_legend_auto3():
     'Test automatic legend placement'
+    # TODO: a slight change in rendering between Inkscape versions may explain
+    # why one had to introduce a small non-zero tolerance for the SVG test
+    # to pass. One may try to remove this hack once Travis' Inkscape version
+    # is modern enough. FWIW, no failure with 0.92.3 on my computer (#11358).
     fig = plt.figure()
     ax = fig.add_subplot(111)
     x = [0.9, 0.1, 0.1, 0.9, 0.9, 0.5]
@@ -539,3 +544,14 @@ def test_draggable():
     with pytest.warns(MatplotlibDeprecationWarning):
         legend.draggable()
     assert not legend.get_draggable()
+
+
+def test_handlerline2d():
+    '''Test consistency of the marker for the (monolithic) Line2D legend
+    handler (see #11357).
+    '''
+    fig, ax = plt.subplots()
+    ax.scatter([0, 1], [0, 1], marker="v")
+    handles = [mlines.Line2D([0], [0], marker="v")]
+    leg = ax.legend(handles, ["Aardvark"], numpoints=1)
+    assert handles[0].get_marker() == leg.legendHandles[0].get_marker()