D3 Collapsible force directed graph with non-tree data - link alignment

Question

If you see the existing code, https://jsfiddle.net/sheilak/9wvmL8q8 when the graph is loaded for first time links that connecting the parent and child node are from border of parent node but once its collapsed and expanded, you can see same links are from center of parent node. i don't want to link to be from center of the parent node.

code

var width = 960,
height = 500;

var svg = d3.select("body").append("svg")
  .attr("width", width)
  .attr("height", height);

var force = d3.layout.force()
  .size([width, height])
  //gravity(0.2)
  .linkDistance(height / 6)
  .charge(function(node) {
    if (node.type !== 'ORG') return -2000;
    return -30;
  });

// build the arrow.
svg.append("svg:defs").selectAll("marker")
  .data(["end"]) // Different link/path types can be defined here
  .enter().append("svg:marker") // This section adds in the arrows
  .attr("id", function(d) {
    return d;
  })
  .attr("viewBox", "0 -5 10 10")
  .attr("refX", 12)
  .attr("refY", 0)
  .attr("markerWidth", 9)
  .attr("markerHeight", 5)
  .attr("orient", "auto")
  .attr("class", "arrow")
  .append("svg:path")
  .attr("d", "M0,-5L10,0L0,5");

  var json = dataset;

  var edges = [];
  json.edges.forEach(function(e) {
    var sourceNode = json.nodes.filter(function(n) {
        return n.id === e.from;
      })[0],
      targetNode = json.nodes.filter(function(n) {
        return n.id === e.to;
      })[0];

    edges.push({
      source: sourceNode,
      target: targetNode,
      value: e.Value
    });
  });

  for(var i = 0; i < json.nodes.length; i++) {
    json.nodes[i].collapsing = 0;
    json.nodes[i].collapsed = false;
  }

  var link = svg.selectAll(".link");
  var node = svg.selectAll(".node");

  force.on("tick", function() {
    // make sure the nodes do not overlap the arrows
    link.attr("d", function(d) {
      // Total difference in x and y from source to target
      diffX = d.target.x - d.source.x;
      diffY = d.target.y - d.source.y;

      // Length of path from center of source node to center of target node
      pathLength = Math.sqrt((diffX * diffX) + (diffY * diffY));

      // x and y distances from center to outside edge of target node
      offsetX = (diffX * d.target.radius) / pathLength;
      offsetY = (diffY * d.target.radius) / pathLength;

      return "M" + d.source.x + "," + d.source.y + "L" + (d.target.x - offsetX) + "," + (d.target.y - offsetY);
    });

    node.attr("transform", function(d) {
      return "translate(" + d.x + "," + d.y + ")";
    });
  });

update();

function update(){
  var nodes = json.nodes.filter(function(d) {
    return d.collapsing == 0;
  });

  var links = edges.filter(function(d) {
    return d.source.collapsing == 0 && d.target.collapsing == 0;
  });

  force
    .nodes(nodes)
    .links(links)
    .start();

  link = link.data(links)

  link.exit().remove();

  link.enter().append("path")
    .attr("class", "link")
    .attr("marker-end", "url(#end)");

  node = node.data(nodes);

  node.exit().remove();

  node.enter().append("g")
    .attr("class", function(d) {
      return "node " + d.type
    });

  node.append("circle")
    .attr("class", "circle")
    .attr("r", function(d) {
      d.radius = 30;
      return d.radius
    }); // return a radius for path to use 

  node.append("text")
    .attr("x", 0)
    .attr("dy", ".35em")
    .attr("text-anchor", "middle")
    .attr("class", "text")
    .text(function(d) {
      return d.type
    });

  // On node hover, examine the links to see if their
  // source or target properties match the hovered node.
  node.on('mouseover', function(d) {
    link.attr('class', function(l) {
      if (d === l.source || d === l.target)
        return "link active";
      else
        return "link inactive";
    });
  });

  // Set the stroke width back to normal when mouse leaves the node.
  node.on('mouseout', function() {
    link.attr('class', "link");
  })
  .on('click', click);

  function click(d) {
    if (!d3.event.defaultPrevented) {
      var inc = d.collapsed ? -1 : 1;
      recurse(d);

      function recurse(sourceNode){
        //check if link is from this node, and if so, collapse
        edges.forEach(function(l) {
          if (l.source.id === sourceNode.id){
            l.target.collapsing += inc;
            recurse(l.target);
          }
        });
      }
      d.collapsed = !d.collapsed;
    }      
    update();
  }
}

Answer 1

There are two simple ways to address this that require little modification of your existing code.

The first is half done as the target of each link is already offset when you define your path data:

  return "M" + d.source.x + "," + d.source.y + "L" + (d.target.x - offsetX) + "," + (d.target.y - offsetY);
});

You could extend this to offset from the source node quite easily, just add the offsets to sourceX and sourceY as here. This way it doesn't matter if the nodes are above or under the links because they don't overlap. (There might be slight overlap, so you could add a pixel or two to the offsets to account for link width).

The second option is possibly easier in d3v4+, as it features selection.raise() (docs). This method raises the selected item to the top of the SVG (as the last child of the parent element). The is equivalent to:

 this.parentNode.appendChild(this);

In your click function, after you update the graph, we can use this line to ensure the node that was clicked on rises to the top (over the links). Here's an example of that.