import_graph(graph_file, file_type = NULL, graph_name = NULL,
graph_time = NULL, graph_tz = NULL)graphml (GraphML) and sif (SIF). If not supplied, the function
will infer the type by its file extension.temporal).tz)
corresponding to the date or date-time string supplied as a value to
graph_time. If no time zone is provided then it will be set to
GMT.dgr_graph.