graph Perl
{
graph [ rankdir="TB" ]
node [ shape="oval" ]
edge [ color="grey" ]
"6" [ color="invis" label="" shape="point" width="0" ]
"20" [ color="invis" label="" shape="point" width="0" ]
"27" [ color="invis" label="" shape="point" width="0" ]
"13" [ color="invis" label="" shape="point" width="0" ]
"6" -- "20"
"20" -- "27"
"27" -- "13"
}