No vertices:
One vertex.
Full graph of 4 vertices, no loops.
4 vertices, disconnected, with loops and multi-edges.
Full graph of 4 vertices, no loops with no repulsion.
4 vertices in a line, with no repulsion, spring length 1 and a seed:
[  1.06066 -1.06066
  0.353553 -0.353553
  -0.353553 0.353553
  -1.06066  1.06066 ]
4 vertices in a line, with no repulsion, spring length 1 and a seed, no sa_movement:
[     0.15    -0.15
      0.05    -0.05
     -0.05     0.05
     -0.15     0.15 ]
Wrong size seed.