The class adds the draw method to the
dg.FactorVertex-class.
The objects of the classes are usually created by the function
returnFactorVerticesAndEdges.
"dg.FactorVertex", directly.
Class "dg.Vertex", directly.
Class "dg.Node", directly.newFactor,
returnFactorVerticesAndEdges,
dg.FactorVertex-class,
dg.Vertex-class,
dg.Node-class.