Hello Abs,

The diagram layout is applied only initially to the diagram. If the items are added/removed later (eg. view model collection), the layout is not automatically applied. In order to rearrange the nodes at a later stage, call the RefreshLayout method directly on the XamDiagram.

eg.

mainWindow.Diagram.RefreshLayout();

For more details please visit:
http://es.infragistics.com/help/wpf/xamdiagram-configuring-the-layout

If you require a custom position you can use the diagram's items collection changed event to do so:

eg.

   private void Items_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
        {
            FamilyTreeViewModel vm = this.Diagram.DataContext as FamilyTreeViewModel;

            if (e.NewStartingIndex >= vm.FamilyTree.Count)
            {
                if (e.NewItems[0] is DiagramNode)
                {

                    DiagramNode newlyAdded = e.NewItems[0] as DiagramNode;
   
                    newlyAdded.Position = new Point(50, 50);
                }
               
            }
        }

Let me know if you have any questions regarding this matter.

http://es.infragistics.com/help/wpf/xamdiagram-configuring-the-layout

Let me know if you have any questions.

http://help.infragistics.com/doc/WPF/2016.1/CLR4.0/?page=InfragisticsWPF4.Controls.Charts.XamDiagram.v16.1~Infragistics.Controls.Charts.XamDiagram_members.html

I have attached an example with this post can you please suggest the right event? I would like to retain TreeDiagramLayout implemented by infragistics and no custom layout is required.

Also, I would like user to be able to move diagram nodes horizontally and restrict any vertical movement in diagram.

The XamDiagram contains an Items collection. This member exposes a CollectionChanged event:

http://es.infragistics.com/help/wpf/infragisticswpf4.controls.charts.xamdiagram.v16.2~infragistics.controls.charts.diagramitemscollection_members

Eg.

Diagram.Items.CollectionChanged += Items_CollectionChanged;

You can handle the diagram's NodeMoving event, create a new Point for the node based on the old Y position and cancel it whenever the Node's Y attempts to change. I've tested this and the nodes move only in a horizontal direction.

eg.

private void Diagram_NodeMoving(object sender, DiagramNodeMovingEventArgs e)
        {
            if (e.OldPosition.Y != e.NewPosition.Y)
            {
                Point nodePoint = new Point(e.NewPosition.X, e.OldPosition.Y);
                e.Node.Position = nodePoint;
                e.Cancel = true;
            }            
        }

Let me know if you have any questions regarding this matter.