Use masked stack to preserve mask info

Author: chahak13

lib/matplotlib/collections.py

@@ -322,7 +322,7 @@ def _prepare_points(self):
                 paths.append(mpath.Path(np.column_stack([xs, ys]), path.codes))
             xs = self.convert_xunits(offsets[:, 0])
             ys = self.convert_yunits(offsets[:, 1])
-            offsets = np.column_stack([xs, ys])
+            offsets = np.ma.column_stack([xs, ys])
 
         if not transform.is_affine:
             paths = [transform.transform_path_non_affine(path)