Support marker="none" to mean "no marker".

As for other APIs, this is less likely to be confused with the None
object.

Author: anntzer

doc/users/next_whats_new/marker_none.rst

@@ -0,0 +1,5 @@
+``marker`` can now be set to the string "none"
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+... to mean *no-marker*, consistently with other APIs which support the
+lowercase version.  Using "none" is recommended over using "None", to avoid
+confusion with the None object.

lib/matplotlib/legend_handler.py

@@ -566,7 +566,7 @@ def create_artists(self, legend, orig_handle,
             self.update_prop(legline, plotlines, legend)
 
             legline.set_drawstyle('default')
-            legline.set_marker('None')
+            legline.set_marker('none')
 
             self.update_prop(legline_marker, plotlines, legend)
             legline_marker.set_linestyle('None')

lib/matplotlib/markers.py

@@ -45,7 +45,8 @@
 ``9`` (``CARETRIGHTBASE``)     |m34|  caretright (centered at base)
 ``10`` (``CARETUPBASE``)       |m35|  caretup (centered at base)
 ``11`` (``CARETDOWNBASE``)     |m36|  caretdown (centered at base)
-``"None"``, ``" "`` or  ``""``        nothing
+``"none"`` or ``"None"``              nothing
+``" "`` or  ``""``                    nothing
 ``'$...$'``                    |m37|  Render the string using mathtext.
                                       E.g ``"$f$"`` for marker showing the
                                       letter ``f``.
@@ -200,6 +201,7 @@ class MarkerStyle:
         CARETUPBASE: 'caretupbase',
         CARETDOWNBASE: 'caretdownbase',
         "None": 'nothing',
+        "none": 'nothing',
         None: 'nothing',
         ' ': 'nothing',
         '': 'nothing'
@@ -227,7 +229,7 @@ def __init__(self, marker=_unset, fillstyle=None):
             - Another instance of *MarkerStyle* copies the details of that
               ``marker``.
             - *None* means no marker.  This is the deprecated default.
-            - For other possible marker values see the module docstring
+            - For other possible marker values, see the module docstring
               `matplotlib.markers`.
 
         fillstyle : str, default: :rc:`markers.fillstyle`

lib/matplotlib/tests/test_axes.py

@@ -4117,8 +4117,9 @@ def test_eventplot_orientation(data, orientation):
 @image_comparison(['marker_styles.png'], remove_text=True)
 def test_marker_styles():
     fig, ax = plt.subplots()
-    for y, marker in enumerate(sorted(matplotlib.markers.MarkerStyle.markers,
-                                      key=lambda x: str(type(x))+str(x))):
+    for y, marker in enumerate(sorted(
+            {*matplotlib.markers.MarkerStyle.markers} - {"none"},
+            key=lambda x: str(type(x))+str(x))):
         ax.plot((y % 2)*5 + np.arange(10)*10, np.ones(10)*10*y, linestyle='',
                 marker=marker, markersize=10+y/5, label=marker)