fix typo (add negation for isAddedWheelEvent statement)

Author: Maksim

src/plots/cartesian/dragbox.js

@@ -1230,7 +1230,7 @@ function attachWheelEventHandler(element, handler) {
     if(!supportsPassive) {
         if(element.onwheel !== undefined) element.onwheel = handler;
         else if(element.onmousewheel !== undefined) element.onmousewheel = handler;
-        else if(element.isAddedWheelEvent) {
+        else if(!element.isAddedWheelEvent) {
             element.isAddedWheelEvent = true;
             element.addEventListener('wheel', handler, {passive: false});
         }