{nodes: [
{id:s1, val:12},]
{id:s2, val:12},
{id:s3, val:12},
{id:s4, val:12},
{id:s5, val:12},
{id:s6, val:12}],
"Links": [{"n1": "s1", "n2": "s2", "amount": 10},
{"n1": "s2", "n2": "s3", "amount": 10},
{"n1": "s2", "n2": "s4", "amount": 10},
{"n1": "s2", "n2": "s6", "amount": 10},
{"n1": "s3", "n2": "s1", "amount": 10},
{"n1": "s4", "n2": "s5", "amount": 10},
{"n1": "s5", "n2": "s6", "amount": 10}]}

{n:s1, num:2, sum:20},
{n:s2, num:4, sum:40},
{n:s3, num:2, sum:20},
{n:s4, num:2, sum:20},
{n:s5, num:2, sum:20},
{n:s6, num:2, sum:20}

var sums = d3.nest()
  .key(function(d) { return d.n1; })
  .rollup(function(v) { return {
    num: v.length,
    sum: d3.sum(v, function(d) { return d.amount; })
  .entries(data.links)