replaced drag selection XOR with more reasonable behavior.

src/drag.js

@@ -47,11 +47,11 @@ c3_chart_internal_fn.drag = function (mouse) {
                 // line/area selection not supported yet
                 return;
             }
-            if (isWithin ^ isIncluded) {
+            if (isWithin ^ isSelected || isWithin ^ isIncluded) {
-                shape.classed(CLASS.INCLUDED, !isIncluded);
+                shape.classed(CLASS.INCLUDED, isWithin);
                 // TODO: included/unincluded callback here
-                shape.classed(CLASS.SELECTED, !isSelected);
+                shape.classed(CLASS.SELECTED, isWithin);
-                toggle.call($$, !isSelected, shape, d, i);
+                toggle.call($$, isWithin, shape, d, i);
             }
         });
 };