Edit in JSFiddle

data_set = [			{name:"aaa"},
                  {name:"bbb"},
                  {name:"ccc"},
                  {name:"ddd"},
                  {name:"eee"},
                  {name:"fff"}  ]  ;
link_set = [
	{source:"aaa", target:"bbb"},
	{source:"bbb", target:"ccc"},
	{source:"ccc", target:"ddd"},
	{source:"ddd", target:"eee"},
	{source:"eee", target:"fff"},
	{source:"fff", target:"aaa"},
];                  
var width=600, height=300;

var force_g = d3.select("#content").append("g") ;

var links =  force_g.selectAll(".link")
	.data(link_set)
  .enter()
  .append("line")
  .attr("class", "link")
  .attr("stroke", "#000")
  .attr("stroke-weight", 1)
  ;

var circles =  force_g.selectAll("circle")
  .data(data_set)
  .enter()
  .append("circle")
  .attr("class","test-circle")
  .attr("r", 5)
  .attr("fill","lightblue")
  .call(d3.drag()
          .on("start", dragstarted)
          .on("drag", dragged)
          .on("end", dragended)
          )
  ;

var line_force = d3.forceSimulation()
	.nodes(data_set)
  .on("tick", ticked)
  .force("link", d3.forceLink(link_set)
  								.id(function(d){ return d.name ; })
                  .distance(20) )
  .force("center", d3.forceCenter(width/2, height/2))
  .force('charge', d3.forceManyBody().strength(15))
  .force("collision", d3.forceCollide(15))
  ;	

function dragstarted(d) {
  if (!d3.event.active) line_force.alphaTarget(0.9).restart();
  d.fx = d.x;
  d.fy = d.y;
}

function dragged(d) {
  d.fx = d3.event.x;
  d.fy = d3.event.y;
}

function dragended(d) {
  if (!d3.event.active) line_force.alphaTarget(0);
  d.fx = null;
  d.fy = null;
 }

function ticked(){
	circles
  	.attr("cx", function(d){ return d.x; })
    .attr("cy", function(d){ return d.y; }) ;
    
  links
  .attr("x1", function(d) { return d.source.x; })
  .attr("y1", function(d) {	return d.source.y; })
  .attr("x2", function(d) {	return d.target.x; })
  .attr("y2", function(d) {	return d.target.y; });
}

 <script src="https://d3js.org/d3.v4.min.js" charset="utf-8"></script>
 <svg id="content" width="640" height="400" ></svg>