Edit in JSFiddle

data_set = [			{name:"aaa",fx:400,fy:200},
                  {name:"bbb"} ]  ;
link_set = [
	{source:"aaa", target:"bbb"},
];    

var width=600, height=300;

d3.select("#content")
	.append("rect")
  .attr("x",380)
  .attr("y",30)
  .attr("width", 40)
  .attr("height",40)
  .attr('fill', 'rgba(0,0,0,0)')
  .attr('stroke', 'red') 
  ;

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", "none")
  .attr("stroke-weight", 1)
  ;

var circles =  force_g.selectAll("circle")
  .data(data_set)
  .enter()
  .append("circle")
  .attr("class",function(d){ return "circle-"+d.name })
  .attr("r", 15)
  .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(0)
                  .strength(0.5))
  .force('charge', d3.forceManyBody().strength(0.8))
  .force("collision", d3.forceCollide(0))
  ;	

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;
  var a = d3.select(".circle-aaa") ;
  a.datum().fy=50;
  a.datum().fx=400;
 }

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>