Graphviz DOT file that generates the image @reinh is looking for in the tweet at https://twitter.com/ReinH/status/440955529494413313 Renders as http://ow.ly/i/4PO8v

pentagonal-graph-for-reinh.gv

//
// Run this graph definition through the dot rendering engine.
//
// E.g.:
//   dot -Tpng pentagonal-graph-for-reinh.gv -o image.png
// or
//   dot -Txlib pentagonal-graph-for-reinh.gv
//
// Also note that if you don't care about the exact orientation,
// the "out of the box" rendering with the circo engine is pretty
// much exactly what you're looking for.
//
digraph {

  // Use straight lines rather than curved:
  splines=false

  // Tweak the width of the graph by setting the distance
  // between nodes of the same rank:
  nodesep=0.4

  // Use text-only nodes:
  node [shape=plaintext]

  // Create the five (visible) nodes:
  a1 [label="a"]
  b1 [label="b"]
  b2 [label="b"]
  a2 [label="a"]
  c

  // ...and the edges between them:
  a1 -> b1
  a1 -> c
  a1 -> a2
  b1 -> b2 [weight=10]
  b2 -> a2
  c -> a2
  a1 -> b2
  b1 -> a2
  // (Note that setting "weight=10" keeps the b1->b2 edge
  // short and straight.)

  // Now we create some hidden nodes and edges to
  // tweak the layout.

  // Set the node and edge styles to be invisible:
  node [shape=point height=0 width=0]
  edge [style=invis]
  // (Note that you can change "invis" to "dotted" to see
  // what's happening here.)

  // Create a couple of dummy nodes (below node a1 and
  // above node c) and ensure they are on the same row
  // (rank) as node b1:
  { rank=same; b1; belowa1; abovec; }

  // Create a couple more dummy nodes, on the same rank
  // as node b2:
  { rank=same; b2; abovea2; belowc; }

  // Create some invisble edges between the real nodes
  // and the dummy nodes to influence placement:
  a1 -> abovec -> c -> belowc -> a2
  a1 -> belowa1 -> abovea2 -> a2
  b1 -> belowa1 -> c
}