public KnowledgeGraphLayerIDSet GetIDSet()

var map = MapView.Active.Map;
if (map.MapType != MapType.LinkChart)
  return;

// find the KG layer
var kgLayer = map.GetLayersAsFlattenedList().OfType<KnowledgeGraphLayer>().FirstOrDefault();
if (kgLayer == null)
  return;

// find the first LinkChartFeatureLayer in the KG layer
var lcFeatureLayer = kgLayer.GetLayersAsFlattenedList().OfType<LinkChartFeatureLayer>().FirstOrDefault();
if (lcFeatureLayer != null)
  return;

QueuedTask.Run(() =>
{
  // get the KG
  var kg = kgLayer.GetDatastore();

  // get the ID set of the KG layer
  var idSet = kgLayer.GetIDSet();

  // get the named object type in the KG that the LinkChartFeatureLayer represents
  var typeName = lcFeatureLayer.GetTypeName();
  // if there are items in the ID Set for this type
  if (idSet.Contains(typeName))
  {
    // build a new ID set containing only these records
    var dict = idSet.ToOIDDictionary();
    var oids = dict[typeName];

    var newDict = new Dictionary<string, List<long>>();
    newDict.Add(typeName, oids);

    var newIDSet = KnowledgeGraphLayerIDSet.FromDictionary(kg, newDict);

    // now create a new link chart using just this subset of records
    MapFactory.Instance.CreateLinkChart("subset LinkChart", kg, newIDSet);
  }
});

QueuedTask.Run(() =>
{
  var idSet = kgLayer.GetIDSet();

  // is the set empty?
  var isEmpty = idSet.IsEmpty;
  // get the count of named object types
  var countNamedObjects = idSet.NamedObjectTypeCount;
  // does it contain the entity "Species";
  var contains = idSet.Contains("Species");

  // get the idSet as a dictionary of namedObjectType and oids
  var oidDict = idSet.ToOIDDictionary();
  var speciesOIDs = oidDict["Species"];

  // alternatively get the idSet as a dictionary of 
  // namedObjectTypes and uuids
  var uuidDict = idSet.ToUIDDictionary();
  var speciesUuids = uuidDict["Species"];

});