One Path for All Links

<!DOCTYPE html>
<meta charset="utf-8">
<style>

.link {
  stroke: #000;
}

</style>
<svg width="960" height="500"></svg>
<script src="//d3js.org/d3.v3.min.js"></script>
<script>

var svg = d3.select("svg"),
    width = +svg.attr("width"),
    height = +svg.attr("height");

d3.json("miserables.json", function(error, graph) {
  if (error) throw error;

var force = d3.layout.force()
      .nodes(graph.nodes)
      .links(graph.links)
      .size([width, height])
      .start();

var link = svg.append("path")
      .attr("class", "link");

var node = svg.selectAll(".node")
      .data(graph.nodes)
    .enter().append("circle")
      .attr("class", "node")
      .attr("r", 4.5)
      .call(force.drag);

force.on("tick", function() {
    link.attr("d", "M" + graph.links.map(function(d) { return d.source.x + "," + d.source.y + "L" + d.target.x + "," + d.target.y; }).join("M"));
    node.attr("transform", function(d) { return "translate(" + d.x + "," + d.y + ")"; });
  });
});

</script>

https://d3js.org/d3.v3.min.js