Must be instantiated at library-level
type Base_Dummy_Vertex is new Vertex with record
Layer : Integer;
end record;
A vertex that is not part of the original graph
procedure Get_Size (V : Vertex_Access; Width, Height : out Gdouble)
returns the size of the vertex
procedure Set_Position (V : Vertex_Access; X, Y : Gdouble)
Set the position of the item