Comment why the offset of the pixel rectangle is surprising.

Author: mdboom

lib/matplotlib/markers.py

@@ -280,6 +280,15 @@ def _set_circle(self, reduction = 1.0):
 
     def _set_pixel(self):
         self._path = Path.unit_rectangle()
+        # Ideally, you'd want -0.5, -0.5 here, but then the snapping
+        # algorithm in the Agg backend will round this to a 2x2
+        # rectangle from (-1, -1) to (1, 1).  By offsetting it
+        # slightly, we can force it to be (0, -1) to (1, 0), which
+        # both makes it only be a single pixel and places it correctly
+        # with 1-width stroking (i.e. the ticks).  This hack is the
+        # best of a number of bad alternatives, mainly because the
+        # backends are not aware of what marker is actually being used
+        # beyond just its path data.
         self._transform = Affine2D().translate(-0.49999, -0.50001)
         self._snap_threshold = None