title: JavaFX group ObservableList by multiple parameters into a map and bind them
body: <p>Assume we have an <code>ObservableList&lt;Person&gt;</code> where Person has properties:age, gender. The goal is to create an observable (at all levels) map similar to
  <code>Map&lt;Gender, Map&lt;Age, List&lt;Person&gt;&gt;&gt;</code> that I can bind to the <code>ObservableList</code>. </p>
  
  <p>The reason for doing this is because I have to extract live aggregate data based on such a grouping.</p>