d3.js - D3 Force Layout Graph - Self linking node

Question

Using a force directed graph, how do you get a link to actually show up when the target and source are the same node. So basically just a nice little loop indicating that such an edge exists.

There are two D3 examples that I already used or tried to use:

• I'm using http://bl.ocks.org/d3noob/5155181 to show direction, and the little endpoint arrow will show up pointing at itself, but no link line.
• http://bl.ocks.org/GerHobbelt/3616279 does does allow for self referencing and I even sort of got it to work with my data, but it is crazy complicated.

Answer 1

The trick is to draw the self link as a path with an arc in it. It took me a bit of fiddling with the arc parameter syntax to get things working and the key seemed to be that the arc could not start and end at the same point. Here is the relevant code that draws the edges at each update.

function tick() {
  link.attr("d", function(d) {
  var x1 = d.source.x,
      y1 = d.source.y,
      x2 = d.target.x,
      y2 = d.target.y,
      dx = x2 - x1,
      dy = y2 - y1,
      dr = Math.sqrt(dx * dx + dy * dy),

      // Defaults for normal edge.
      drx = dr,
      dry = dr,
      xRotation = 0, // degrees
      largeArc = 0, // 1 or 0
      sweep = 1; // 1 or 0

      // Self edge.
      if ( x1 === x2 && y1 === y2 ) {
        // Fiddle with this angle to get loop oriented.
        xRotation = -45;

        // Needs to be 1.
        largeArc = 1;

        // Change sweep to change orientation of loop. 
        //sweep = 0;

        // Make drx and dry different to get an ellipse
        // instead of a circle.
        drx = 30;
        dry = 20;

        // For whatever reason the arc collapses to a point if the beginning
        // and ending points of the arc are the same, so kludge it.
        x2 = x2 + 1;
        y2 = y2 + 1;
      } 

 return "M" + x1 + "," + y1 + "A" + drx + "," + dry + " " + xRotation + "," + largeArc + "," + sweep + " " + x2 + "," + y2;
});

And here is a jsfiddle that demonstrates the whole thing, and a screenshot: