Helvetica in `guix graph`

[Fulbert]

  Hi Guixers,

  When I pipe `guix graph hello | xdot -`, the characters are
displayed as squares. Got to insert
`… | sed "s/Helvetica/sans/g" | …` in the pipeline.

  Not sure if this is a bug. Maybe my system should find a
substitute for "Helvetica" automatically ?

  … or maybe line 241 of 'guix/graph.scm' needs a tiny correction ?
----
(format port "  \"~a\" [label = \"~a\", shape = box, fontname = Helvetica];~%" 
id label))
----

  Best regards,
  Fulbert