Control spacing in twopi (Graphviz)

I'm a newbie with Graphviz, and I'm trying to draw a tree centered in an entity (I'm using the `twopi` command). - If I put `overlap=true` it overlaps even if it has a lot of space aroud the overlapped labels. - If I put `overlap=false`, labels become too small. How can I have the first situation without overlapping? Complete code: ``` digraph g { graph [ fontname = "Helvetica", fontsize = 10, size = "500,500", splines=true, overlap=false, ratio=.5 ]; node [ shape = plaintext, fontname = "Helvetica" ]; root="owl:Thing"; "owl:Thing" -> "Work"; "Work" -> "WrittenWork"; "Work" -> "Software"; "Work" -> "Website"; "Work" -> "Film"; "owl:Thing" -> "Agent"; "Agent" -> "Organisation"; "Organisation" -> "Non-ProfitOrganisation"; "Organisation" -> "GeopoliticalOrganisation"; "Organisation" -> "SambaSchool"; "Agent" -> "Person"; "Person" -> "Athlete"; "Person" -> "OfficeHolder"; "Person" -> "Astronaut"; "Person" -> "Philosopher"; "Person" -> "Architect"; "owl:Thing" -> "Drug"; "owl:Thing" -> "Place"; "Place" -> "SiteOfSpecialScientificInterest"; "Place" -> "PopulatedPlace"; "PopulatedPlace" -> "Country"; "PopulatedPlace" -> "Continent"; "PopulatedPlace" -> "Atoll"; "Place" -> "ProtectedArea"; "Place" -> "ArchitecturalStructure"; "Place" -> "HistoricPlace"; "Place" -> "NaturalPlace"; "NaturalPlace" -> "Mountain"; "NaturalPlace" -> "Volcano"; "NaturalPlace" -> "MountainRange"; } ``` Thank you, Alessio