如何使d3.js中节点的距离强制布局更大？

我试图用d3js强制布局表示我的数据。我有以下代码：

I am trying to represent my data with d3js force layout. I have the following code:

<script>
 var svg = d3.select("svg"),
      width = +svg.attr("width"),
      height = +svg.attr("height");
 var color = d3.scaleOrdinal(d3.schemeCategory20);

 var simulation = d3.forceSimulation()
      .force("link", d3.forceLink().id(function(d) { return d.id; }))
      .force("charge", d3.forceManyBody())
      .force("center", d3.forceCenter(width / 2, height / 2));

 var jsonData = JSON.parse(data);
 var totalAmount=jsonData.total;
 d3.json("myData.json", function(error, graph) {
 if (error) throw error;
 var nodes = json.nodes;
 var link = svg.append("g")
        .attr("class", "links")
        .selectAll("line")
        .data(graph.links)
        .enter().append("line")
        //.attr("stroke-width", function(d) { return Math.sqrt(d.value); });
        .attr("stroke-width",4)

var node = svg.append("g")
        .attr("class", "nodes")
        .selectAll("circle")
        .data(graph.nodes)
        .enter().append("circle")
        .attr("r", 5)
        .attr("fill", function(d) { return color(d.group); })
        .call(d3.drag()
                .on("start", dragstarted)
                .on("drag", dragged)
                .on("end", dragended));

node.append("title")
        .text(function(d) { return d.id; });

simulation
        .nodes(graph.nodes)
        .on("tick", ticked);

simulation.force("link")
        .links(graph.links);

function ticked() {
  link
          .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; });

  node
          .attr("cx", function(d) { return d.x; })
          .attr("cy", function(d) { return d.y; });
    }
  });

  function dragstarted(d) {
     if (!d3.event.active) simulation.alphaTarget(0.3).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) simulation.alphaTarget(0);
      d.fx = null;
      d.fy = null;
   }

然而，在输出中，节点彼此如此接近，此照片：
我的结果

However, in the output, the nodes are so close to each others like in this photo: My result

我检查了链接是否正确（基本上我有一个源和如此多的目标节点。）

I checked that links are correct (basically I have one source and so many target nodes.)

如何使节点之间的距离更大？非常感谢您的帮助。

How can I make larger the distance between nodes? Thanks for any help.