graph.json

{
  "nodes": [
    {"x": 480, "y": 250, "group": 0, "radius": 50},
    {"x": 240, "y": 250, "group": 1, "radius": 5},
    {"x": 720, "y": 250, "group": 2, "radius": 5}
  ],
  "links": [
    {"source":  1, "target":  0},
    {"source":  2, "target":  0}
  ]
}