Problem:
Memory leak occur in Shadow.NONE. Because all listeners are never removed.

Solutions:

• Shadow.NONE can't accept listeners.
• Remove Shadow.NONE and always create a new EmptyShadow instance.

Workaround

chart.getChart().setShadow( new Shadow()
{
      @Override
      public String toJSONString()
      {
          return "false";
      }
} );