Merge pull request matplotlib#5603 from mdboom/circle-marker-snapping

FIX: Draw markers around center of pixels

Author: tacaswell

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

@@ -516,6 +516,7 @@ inline void RendererAgg::draw_markers(GCAgg &gc,
 
     // Deal with the difference in y-axis direction
     marker_trans *= agg::trans_affine_scaling(1.0, -1.0);
+
     trans *= agg::trans_affine_scaling(1.0, -1.0);
     trans *= agg::trans_affine_translation(0.5, (double)height + 0.5);
 
@@ -527,6 +528,13 @@ inline void RendererAgg::draw_markers(GCAgg &gc,
                                points_to_pixels(gc.linewidth));
     curve_t marker_path_curve(marker_path_snapped);
 
+    if (!marker_path_snapped.is_snapping()) {
+        // If the path snapper isn't in effect, at least make sure the marker
+        // at (0, 0) is in the center of a pixel.  This, importantly, makes
+        // the circle markers look centered around the point they refer to.
+        marker_trans *= agg::trans_affine_translation(0.5, 0.5);
+    }
+
     transformed_path_t path_transformed(path, trans);
     nan_removed_t path_nan_removed(path_transformed, false, false);
     snap_t path_snapped(path_nan_removed, SNAP_FALSE, path.total_vertices(), 0.0);