D3 Force Layout : How to force a group of node to stay in a given area

In a D3 force layout graph, i try to force certain nodes to stay in a given area, based on their group.

enter image description here

There is a center node, which is fixed. I want nodes connected by red line to stay in first 1/3 of screen height, nodes connected by stroke line stay in the second 1/3 and nodes connected by blue lines stay in last 1/3.

But i want it smooth : at the loading, nodes moves to the given zone, and stay away from the other. The distance with the center node may not be fixed.

The number of nodes in each group is variable.

How can achieve that with d3.js v4 ?

Thanks for help :)

Answers:

Answer

You have to use forceX, which:

Creates a new positioning force along the x-axis towards the given position x. If x is not specified, it defaults to 0.

In this demo (based on this Bostock's code), the nodes array contains a group:

nodes:[
    {"id": "A", "group": 1},
    {"id": "B", "group": 2},
    {"id": "C", "group": 2},
    //etc...
]

Using this group value, we can position the nodes to the left or to the right. This is just an example, change it according to your code:

.force("x", d3.forceX(function(d){
    if(d.group === 2){
        return width/3
    } else if (d.group === 3){
        return 2*width/3
    } else {
        return width/2 
    }
}))

So, if the nodes belongs to group 2 (cyan nodes), we position it at 1/3 of the width, and if it belongs to group 3 (orange nodes) we position it at 2/3 of the width.

The positioning is not perfect, because other parameters can influence it, as the charge's strength and the link's distance.

This is a demo:

var graph = {
nodes:[
	{"id": "A", "group": 1},
	{"id": "B", "group": 2},
	{"id": "C", "group": 2},
	{"id": "D", "group": 2},
	{"id": "E", "group": 2},
	{"id": "F", "group": 3},
	{"id": "G", "group": 3},
	{"id": "H", "group": 3},
	{"id": "I", "group": 3}
],
links:[
{"source": "A", "target": "B", "value": 1},
{"source": "A", "target": "C", "value": 1},
{"source": "A", "target": "D", "value": 1},
{"source": "A", "target": "E", "value": 1},
{"source": "A", "target": "F", "value": 1},
{"source": "A", "target": "G", "value": 1},
{"source": "A", "target": "H", "value": 1},
{"source": "A", "target": "I", "value": 1},
]
};

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; }).distance(100))
    .force("charge", d3.forceManyBody())
		.force("x", d3.forceX(function(d){
			if(d.group === 2){
				return width/3
			} else if (d.group === 3){
				return 2*width/3
			} else {
				return width/2 
			}
		}))
    .force("y", d3.forceY(height/2))
    .force("center", d3.forceCenter(width / 2, height / 2));

  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); });

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

.nodes circle {
  stroke: #fff;
  stroke-width: 1.5px;
}
<svg width="400" height="300"></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>

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