drifting force directed graph

This force graph drifts while the simulation is running.

What happens is that every tick adds 1 to the x and y position of the centering force, so the graph slowly drifts off the screen.

Click here for the full explanation.

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

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

.nodes circle {
  stroke: black	;
  stroke-width: 0px;
}

</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": "Lillian", "sex": "F"},
    {"name": "Gordon", "sex": "M"},
    {"name": "Sylvester", "sex": "M"},
    {"name": "Mary", "sex": "F"},
    {"name": "Helen", "sex": "F"},
    {"name": "Jamie", "sex": "M"},
    {"name": "Jessie", "sex": "F"},
    {"name": "Ashton", "sex": "M"},
    {"name": "Duncan", "sex": "M"},
    {"name": "Evette", "sex": "F"},
    {"name": "Mauer", "sex": "M"},
    {"name": "Fray", "sex": "F"},
    {"name": "Duke", "sex": "M"},
    {"name": "Baron", "sex": "M"},
    {"name": "Infante", "sex": "M"},
    {"name": "Percy", "sex": "M"},
    {"name": "Cynthia", "sex": "F"}
    ]

//Sample links data 
//type: A for Ally, E for Enemy
var links_data = [
	{"source": "Sylvester", "target": "Gordon", "type":"A" },
    {"source": "Sylvester", "target": "Lillian", "type":"A" },
    {"source": "Sylvester", "target": "Mary", "type":"A"},
    {"source": "Sylvester", "target": "Jamie", "type":"A"},
    {"source": "Sylvester", "target": "Jessie", "type":"A"},
    {"source": "Sylvester", "target": "Helen", "type":"A"},
    {"source": "Helen", "target": "Gordon", "type":"A"},
    {"source": "Mary", "target": "Lillian", "type":"A"},
    {"source": "Ashton", "target": "Mary", "type":"A"},
    {"source": "Duncan", "target": "Jamie", "type":"A"},
    {"source": "Gordon", "target": "Jessie", "type":"A"},
    {"source": "Sylvester", "target": "Fray", "type":"E"},
    {"source": "Fray", "target": "Mauer", "type":"A"},
    {"source": "Fray", "target": "Cynthia", "type":"A"},
    {"source": "Fray", "target": "Percy", "type":"A"},
    {"source": "Percy", "target": "Cynthia", "type":"A"},
    {"source": "Infante", "target": "Duke", "type":"A"},
    {"source": "Duke", "target": "Gordon", "type":"A"},
    {"source": "Duke", "target": "Sylvester", "type":"A"},
    {"source": "Baron", "target": "Duke", "type":"A"},
    {"source": "Baron", "target": "Sylvester", "type":"E"},
    {"source": "Evette", "target": "Sylvester", "type":"E"},
    {"source": "Cynthia", "target": "Sylvester", "type":"E"},
    {"source": "Cynthia", "target": "Jamie", "type":"E"},
    {"source": "Mauer", "target": "Jessie", "type":"E"}
]

//set up the simulation 
var simulation = d3.forceSimulation()
					//add nodes
					.nodes(nodes_data);
  
var link_force =  d3.forceLink(links_data)
                        .id(function(d) { return d.name; });
                    
var charge_force = d3.forceManyBody()
    .strength(-200);
    
var center_x = 100;
var center_y = 100; 
var center_force = d3.forceCenter()
	.x(center_x)
	.y(center_y);

simulation
    .force("charge_force", charge_force)
    .force("center_force", center_force)
    .force("links",link_force);

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

//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)
      .style("stroke", linkColour);

//draw circles for the nodes 
var node = svg.append("g")
        .attr("class", "nodes") 
        .selectAll("circle")
        .data(nodes_data)
        .enter()
        .append("circle")
        .attr("r", 15)
        .attr("fill", circleColour);

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)

/** Functions **/

//Function to choose what color circle we have
//Let's return blue for males and red for females
function circleColour(d){
	if(d.sex =="M"){
		return "blue";
	} else {
		return "pink";
	}
}

//Function to choose the line colour and thickness 
//If the link type is "A" return green 
//If the link type is "E" return red 
function linkColour(d){
	if(d.type == "A"){
		return "green";
	} else {
		return "red";
	}
}

//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