dragable force directed graphs

Dragable force directed graph. Instructions on the dragging mechanism are here.

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

.links line {
  stroke: #999;
  stroke-opacity: 0.6;
}

.nodes circle {
  stroke: #fff;
  stroke-width: 1.5px;
}

</style>
<svg width="960" height="600"></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>
//create somewhere to put the force directed graph
var svg = d3.select("svg"),
    width = +svg.attr("width"),
    height = +svg.attr("height");
    
    
var nodes_data =  [
    {"name": "Travis", "sex": "M"},
    {"name": "Rake", "sex": "M"},
    {"name": "Diana", "sex": "F"},
    {"name": "Rachel", "sex": "F"},
    {"name": "Shawn", "sex": "M"},
    {"name": "Emerald", "sex": "F"}
    ]

var links_data = [
	{"source": "Travis", "target": "Rake"},
    {"source": "Diana", "target": "Rake"},
    {"source": "Diana", "target": "Rachel"},
    {"source": "Rachel", "target": "Rake"},
    {"source": "Rachel", "target": "Shawn"},
    {"source": "Emerald", "target": "Rachel"}
]

//set up the simulation 
var simulation = d3.forceSimulation()
					//add nodes
					.nodes(nodes_data);
                    
                    
//add forces
//we're going to add a charge to each node 
//also going to add a centering force
//and a link force
var link_force =  d3.forceLink(links_data)
                        .id(function(d) { return d.name; });
simulation
    .force("charge_force", d3.forceManyBody())
    .force("center_force", d3.forceCenter(width / 2, height / 2))
    .force("links",link_force);

//add tick instructions: 
simulation.on("tick", tickActions );

//draw circles for the links 
var node = svg.append("g")
        .attr("class", "nodes")
        .selectAll("circle")
        .data(nodes_data)
        .enter()
        .append("circle")
        .attr("r", 10)
        .attr("fill", "red");

//draw lines for the links 
var link = svg.append("g")
      .attr("class", "links")
    .selectAll("line")
    .data(links_data)
    .enter().append("line")
      .attr("stroke-width", 2);        
                
      
var drag_handler = d3.drag()
	.on("start", drag_start)
	.on("drag", drag_drag)
	.on("end", drag_end);	
	
//same as using .call on the node variable as in https://bl.ocks.org/mbostock/4062045 
drag_handler(node)

//drag handler
//d is the node 
function drag_start(d) {
 if (!d3.event.active) simulation.alphaTarget(0.3	).restart();
   d.fx = d.x;
   d.fy = d.y;
}

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

</script>

https://d3js.org/d3.v4.min.js