[boost] [bgl] write_graphviz without vertex_id

Hello, I have a graph that does not have vertex_id. Each node in my graph might be uniquely identified, because I have a property map to some objects which have a unique id. However, when I try to write the graph to graphviz, it requires this vertex_id property. I bother me to add a vertex_id property to my graph just for this purpose, and I wonder if it could be possible to extend the write_graphviz in order to allow the user to customize the vertex id that is written in the dot file. Regards, -- Loïc