public enum IDSetCombinationMethod : System.Enum, System.IComparable, System.IConvertible, System.IFormattable, System.ISpanFormattable  

{
  if (!MapView.Active?.Map?.IsLinkChart == true)
    return;

  await QueuedTask.Run(() =>
  {
    var map = MapView.Active.Map;
    var sel_set = map.GetSelection();
    if (sel_set.Count > 0)
    {
      var kg_layer_id_set = KnowledgeGraphLayerIDSet.FromSelectionSet(sel_set);
      //IDSetCombinationMethod can be New, Add, or Subtract
      if (map.CanSetLinkChartContent(kg_layer_id_set, IDSetCombinationMethod.New))
      {
        map.SetLinkChartContent(kg_layer_id_set, IDSetCombinationMethod.New);
      }
    }
  });
}

{
  if (!MapView.Active?.Map?.IsLinkChart == true)
    return;

  //Assume we have a set of features in an entity type 'Species' and an entity
  //type 'Observation' that we want to append to an existing link chart...
  //Create a dictionary containing the entity uids to be added...
  var dict = new Dictionary<string, List<object>>();
  dict["Species"] = new List<object>();
  dict["Observation"] = new List<object>();

  dict["Species"].Add("{4478D960-A4D6-4930-A151-51A2C6140509}");
  dict["Species"].Add("{97E011A0-C140-4EA9-84E6-19413F6FCEE3}");
  dict["Observation"].Add("{18E1DB21-9F61-4CBE-A488-71AC8B24B605}");
  dict["Observation"].Add("{E378F1B1-FF5E-4EDD-808A-1C395A4857AD}");

  await QueuedTask.Run(() =>
  {
    var map = MapView.Active.Map;

    //Get the knowledge graph layer from the link chart
    var kg_layer = MapView.Active?.Map?.GetLayersAsFlattenedList()?
                              .OfType<ArcGIS.Desktop.Mapping.KnowledgeGraphLayer>()?
                              .First();

    //arbitrarily get the first feature layer that is a child of the kg layer
    var fl = kg_layer?.GetLayersAsFlattenedList()?
                .OfType<FeatureLayer>()?.First();

    //Get the kg from the kg layer
    using (var kg = kg_layer.GetDatastore())
    {

      //Create a kg layer id set
      var id_set = KnowledgeGraphIDSet.FromDictionary(kg, dict);
      var kg_layer_id_set = KnowledgeGraphLayerIDSet.FromKnowledgeGraphIDSet(id_set);

      //set the content of the link chart using the id set and the Add combination method
      if (map.CanSetLinkChartContent(kg_layer_id_set, IDSetCombinationMethod.Add))
      {
        map.SetLinkChartContent(kg_layer_id_set, IDSetCombinationMethod.Add);
        //Set the newly added records selected...
        map.SetSelection(SelectionSet.FromKnowledgeGraphLayerIDSet(map, kg_layer_id_set));
      }
    }
  });
}

{
  if (!MapView.Active?.Map?.IsLinkChart == true)
    return;

  await QueuedTask.Run(() =>
  {
    var map = MapView.Active.Map;
    var sel_set = map.GetSelection();
    if (sel_set.Count > 0)
    {
      var kg_layer_id_set = KnowledgeGraphLayerIDSet.FromSelectionSet(sel_set);
      //IDSetCombinationMethod can be New, Add, or Subtract
      if (map.CanSetLinkChartContent(kg_layer_id_set, IDSetCombinationMethod.Subtract))
      {
        map.SetLinkChartContent(kg_layer_id_set, IDSetCombinationMethod.Subtract);
      }
    }
  });
}