scatter: fix marker kwarg bug. Closes #4073, #3895.

I also tried to clean up some of the scatter code and documentation,
together with related documentation in collections.  I added a test.

lib/matplotlib/axes/_axes.py

@@ -3511,7 +3511,8 @@ def dopatch(xs, ys, **kwargs):
     @docstring.dedent_interpd
     def scatter(self, x, y, s=20, c='b', marker='o', cmap=None, norm=None,
                 vmin=None, vmax=None, alpha=None, linewidths=None,
-                verts=None, **kwargs):
+                verts=None, edgecolors=None,
+                **kwargs):
         """
         Make a scatter plot of x vs y, where x and y are sequence like objects
         of the same lengths.
@@ -3531,11 +3532,14 @@ def scatter(self, x, y, s=20, c='b', marker='o', cmap=None, norm=None,
             (see below). Note that `c` should not be a single numeric RGB or
             RGBA sequence because that is indistinguishable from an array of
             values to be colormapped.  `c` can be a 2-D array in which the
-            rows are RGB or RGBA, however.
+            rows are RGB or RGBA, however, including the case of a single
+            row to specify the same color for all points.
 
         marker : `~matplotlib.markers.MarkerStyle`, optional, default: 'o'
             See `~matplotlib.markers` for more information on the different
-            styles of markers scatter supports.
+            styles of markers scatter supports. `marker` can be either
+            an instance of the class or the text shorthand for a particular
+            marker.
 
         cmap : `~matplotlib.colors.Colormap`, optional, default: None
             A `~matplotlib.colors.Colormap` instance or registered name.
@@ -3557,10 +3561,14 @@ def scatter(self, x, y, s=20, c='b', marker='o', cmap=None, norm=None,
             The alpha blending value, between 0 (transparent) and 1 (opaque)
 
         linewidths : scalar or array_like, optional, default: None
-            If None, defaults to (lines.linewidth,).  Note that this is a
-            tuple, and if you set the linewidths argument you must set it as a
-            sequence of floats, as required by
-            `~matplotlib.collections.RegularPolyCollection`.
+            If None, defaults to (lines.linewidth,).
+
+        edgecolors : color or sequence of color, optional, default: None
+            If None, defaults to (patch.edgecolor).
+            If 'face', the edge color will always be the same as
+            the face color.  If it is 'none', the patch boundary will not
+            be drawn.  For non-filled markers, the `edgecolors` kwarg
+            is ignored; color is determined by `c`.
 
         Returns
         -------
@@ -3598,43 +3606,41 @@ def scatter(self, x, y, s=20, c='b', marker='o', cmap=None, norm=None,
 
         s = np.ma.ravel(s)  # This doesn't have to match x, y in size.
 
-        c_is_stringy = is_string_like(c) or is_sequence_of_strings(c)
-        if not c_is_stringy:
-            c = np.asanyarray(c)
-            if c.size == x.size:
-                c = np.ma.ravel(c)
+        # After this block, c_array will be None unless
+        # c is an array for mapping.  The potential ambiguity
+        # with a sequence of 3 or 4 numbers is resolved in
+        # favor mapping, not rgb or rgba.
+        try:
+            c_array = np.asanyarray(c, dtype=float)
+            if c_array.shape == x.shape:
+                c = np.ma.ravel(c_array)
+            else:
+                # Wrong shape; it must not be intended for mapping.
+                c_array = None
+        except ValueError:
+            # Failed to make a floating-point array; c must be color specs.
+            c_array = None
+
+        if c_array is None:
+            colors = c     # must be acceptable as PathCollection facecolors
+        else:
+            colors = None  # use cmap, norm after collection is created
 
+        # c will be unchanged unless it is the same length as x:
         x, y, s, c = cbook.delete_masked_points(x, y, s, c)
 
         scales = s   # Renamed for readability below.
 
-        if c_is_stringy:
-            colors = mcolors.colorConverter.to_rgba_array(c, alpha)
-        else:
-            # The inherent ambiguity is resolved in favor of color
-            # mapping, not interpretation as rgb or rgba:
-            if c.size == x.size:
-                colors = None  # use cmap, norm after collection is created
-            else:
-                colors = mcolors.colorConverter.to_rgba_array(c, alpha)
-
-        faceted = kwargs.pop('faceted', None)
-        edgecolors = kwargs.get('edgecolors', None)
-        if faceted is not None:
-            cbook.warn_deprecated(
-                '1.2', name='faceted', alternative='edgecolor',
-                obj_type='option')
-            if faceted:
-                edgecolors = None
-            else:
-                edgecolors = 'none'
-
         # to be API compatible
         if marker is None and not (verts is None):
             marker = (verts, 0)
             verts = None
 
-        marker_obj = mmarkers.MarkerStyle(marker)
+        if isinstance(marker, mmarkers.MarkerStyle):
+            marker_obj = marker
+        else:
+            marker_obj = mmarkers.MarkerStyle(marker)
+
         path = marker_obj.get_path().transformed(
             marker_obj.get_transform())
         if not marker_obj.is_filled():
@@ -3649,9 +3655,9 @@ def scatter(self, x, y, s=20, c='b', marker='o', cmap=None, norm=None,
                 linewidths=linewidths,
                 offsets=offsets,
                 transOffset=kwargs.pop('transform', self.transData),
+                alpha=alpha
                 )
         collection.set_transform(mtransforms.IdentityTransform())
-        collection.set_alpha(alpha)
         collection.update(kwargs)
 
         if colors is None:

lib/matplotlib/collections.py

@@ -559,13 +559,13 @@ def set_color(self, c):
     def set_facecolor(self, c):
         """
         Set the facecolor(s) of the collection.  *c* can be a
-        matplotlib color arg (all patches have same color), or a
-        sequence of rgba tuples; if it is a sequence the patches will
+        matplotlib color spec (all patches have same color), or a
+        sequence of specs; if it is a sequence the patches will
         cycle through the sequence.
 
         If *c* is 'none', the patch will not be filled.
 
-        ACCEPTS: matplotlib color arg or sequence of rgba tuples
+        ACCEPTS: matplotlib color spec or sequence of specs
         """
         self._is_filled = True
         try:
@@ -596,15 +596,15 @@ def get_edgecolor(self):
     def set_edgecolor(self, c):
         """
         Set the edgecolor(s) of the collection. *c* can be a
-        matplotlib color arg (all patches have same color), or a
-        sequence of rgba tuples; if it is a sequence the patches will
+        matplotlib color spec (all patches have same color), or a
+        sequence of specs; if it is a sequence the patches will
         cycle through the sequence.
 
         If *c* is 'face', the edge color will always be the same as
         the face color.  If it is 'none', the patch boundary will not
         be drawn.
 
-        ACCEPTS: matplotlib color arg or sequence of rgba tuples
+        ACCEPTS: matplotlib color spec or sequence of specs
         """
         self._is_stroked = True
         try:

lib/matplotlib/tests/test_axes.py

@@ -16,6 +16,7 @@
 import matplotlib
 from matplotlib.testing.decorators import image_comparison, cleanup
 import matplotlib.pyplot as plt
+import matplotlib.markers as mmarkers
 from numpy.testing import assert_array_equal
 import warnings
 
@@ -1091,7 +1092,24 @@ def test_hist2d_transpose():
 @image_comparison(baseline_images=['scatter'])
 def test_scatter_plot():
     ax = plt.axes()
-    ax.scatter([3, 4, 2, 6], [2, 5, 2, 3], c=['r', 'y', 'b', 'lime'], s=[24, 15, 19, 29])
+    ax.scatter([3, 4, 2, 6], [2, 5, 2, 3],
+               c=['r', 'y', 'b', 'lime'], s=[24, 15, 19, 29])
+
+
+@image_comparison(baseline_images=['scatter_marker'], remove_text=True,
+                  extensions=['png'])
+def test_scatter_marker():
+    fig, (ax0, ax1) = plt.subplots(ncols=2)
+    ax0.scatter([3, 4, 2, 6], [2, 5, 2, 3],
+                c=[(1, 0, 0), 'y', 'b', 'lime'],
+                s=[60, 50, 40, 30],
+                edgecolors=['k', 'r', 'g', 'b'],
+                marker='s')
+    ax1.scatter([3, 4, 2, 6], [2, 5, 2, 3],
+                c=[(1, 0, 0), 'y', 'b', 'lime'],
+                s=[60, 50, 40, 30],
+                edgecolors=['k', 'r', 'g', 'b'],
+                marker=mmarkers.MarkerStyle('o', fillstyle='top'))
 
 
 @cleanup