Merge pull request matplotlib#4422 from tacaswell/fix_circle_marker_snap

FIX : turn path snapping off on 'o' marker path

Author: efiring

doc/api/api_changes/2015-05-12_circle_marker_snap.rst

@@ -0,0 +1,9 @@
+Changed snap threshold for circle markers to inf
+````````````````````````````````````````````````
+
+When drawing circle markers above some marker size (previously 6.0)
+the path used to generate the marker was snapped to pixel centers.  However,
+this ends up distorting the marker away from a circle.  By setting the
+snap threshold to inf snapping is never done on circles.
+
+This change broke several tests, but is an improvement.

lib/matplotlib/markers.py

@@ -346,7 +346,7 @@ def _half_fill(self):
 
     def _set_circle(self, reduction=1.0):
         self._transform = Affine2D().scale(0.5 * reduction)
-        self._snap_threshold = 6.0
+        self._snap_threshold = np.inf
         fs = self.get_fillstyle()
         if not self._half_fill():
             self._path = Path.unit_circle()

lib/matplotlib/tests/baseline_images/test_axes/marker_edges.png

@@ -3586,6 +3586,18 @@ def test_set_get_ticklabels():
     ax[1].set_yticklabels(ax[0].get_yticklabels() )
 
 
+@image_comparison(baseline_images=['o_marker_path_snap'], extensions=['png'],
+                  savefig_kwarg={'dpi': 72})
+def test_o_marker_path_snap():
+    fig, ax = plt.subplots()
+    ax.margins(.1)
+    for ms in range(1, 15):
+        ax.plot([1, 2, ], np.ones(2) + ms, 'o', ms=ms)
+
+    for ms in np.linspace(1, 10, 25):
+        ax.plot([3, 4, ], np.ones(2) + ms, 'o', ms=ms)
+
+
 @cleanup
 def test_margins():
     # test all ways margins can be called