Set data on midpoint circles

js/id/svg/midpoints.js

@@ -47,6 +47,9 @@ iD.svg.Midpoints = function(projection) {
 
         groups.attr('transform', iD.svg.PointTransform(projection));
 
+        groups.selectAll('circle')
+            .data(_.values(midpoints), function (d) { return d.id; });
+
         groups.exit()
             .remove();
     };