Property Value
Within the same layer, nodes with larger ZOrder values are placed in front of nodes with smaller ZOrder values. When the value is NaN the ordering is not specified. The default value is NaN. The value is actually stored on the VisualElement. One normally sets it by setting or data-binding the attached property on the root element of the DataTemplate.
When a Group has a ZOrder value of NaN, it is automatically placed behind its member nodes and links that also have no ZOrder. Such automatic ordering is not guaranteed if any nodes including the groups have a numeric ZOrder. If you do want to specify the ZOrder of nodes, you should also specify the ZOrder of their containing groups.