wpf listcollectionview grouping by multiple columns

// reference: http://stackoverflow.com/a/13017748/1768303

// model
public class Person
{
   public string FirstName { get; set; }
   public string LastName { get; set; }
   public string City { get; set; }
}

// ViewModel
List<Person> people = new List<Person>
{
   new Person { FirstName = "John", LastName = "Doe", City = "London"},
   new Person { FirstName = "John", LastName = "Smith", City = "New York" },
   new Person { FirstName = "Jane", LastName = "Smith", City = "London" },
   new Person { FirstName = "John", LastName = "Smith", City = "Paris" },
};

ListCollectionView view = new ListCollectionView(people);

view.GroupDescriptions.Add(new PropertyGroupDescription("FirstName"));
view.GroupDescriptions.Add(new PropertyGroupDescription("LastName"));

// View
<ListBox x:Name="my