linking nodes of variable radius with arrows

Question

I have some circles/nodes of different radius and I have to connect them with paths having arrow ends.

Here's the code for the marker:

svg.append("svg:defs").selectAll("marker")
    .data(["default"])
  .enter().append("svg:marker")
    .attr("id", String)
    .attr("viewBox", "0 -5 10 10")
    .attr("refX", 5)
    .attr("refY", -1.5)
    .attr("markerWidth", 10)
    .attr("markerHeight", 10)
    .attr("orient", "auto")
    .append("svg:path")
    .attr("d", "M1,-5L10,0L0,5");  

I have stored the radius of circles in an array. Here's the screen shot:

The arrow is actually "inside" the circles. How do I get the arrows to be at the surface of the circles?

Answer 1

This is an old question, but here is my solution if you want your arrowheads to be at the edge of your nodes instead of on top of or beneath them. My approach was also to draw the path connecting the nodes such that the end points were on the nodes' edges rather than at the nodes' centers. Starting from the Mobile Patent Suits example (http://bl.ocks.org/mbostock/1153292), I replaced the linkArc method with:

function linkArc(d) {
    var sourceX = d.source.x;
    var sourceY = d.source.y;
    var targetX = d.target.x;
    var targetY = d.target.y;

    var theta = Math.atan((targetX - sourceX) / (targetY - sourceY));
    var phi = Math.atan((targetY - sourceY) / (targetX - sourceX));

    var sinTheta = d.source.r * Math.sin(theta);
    var cosTheta = d.source.r * Math.cos(theta);
    var sinPhi = d.target.r * Math.sin(phi);
    var cosPhi = d.target.r * Math.cos(phi);

    // Set the position of the link's end point at the source node
    // such that it is on the edge closest to the target node
    if (d.target.y > d.source.y) {
        sourceX = sourceX + sinTheta;
        sourceY = sourceY + cosTheta;
    }
    else {
        sourceX = sourceX - sinTheta;
        sourceY = sourceY - cosTheta;
    }

    // Set the position of the link's end point at the target node
    // such that it is on the edge closest to the source node
    if (d.source.x > d.target.x) {
        targetX = targetX + cosPhi;
        targetY = targetY + sinPhi;    
    }
    else {
        targetX = targetX - cosPhi;
        targetY = targetY - sinPhi;   
    }

    // Draw an arc between the two calculated points
    var dx = targetX - sourceX,
        dy = targetY - sourceY,
        dr = Math.sqrt(dx * dx + dy * dy);
    return "M" + sourceX + "," + sourceY + "A" + dr + "," + dr + " 0 0,1 " + targetX + "," + targetY;
}

Note that this code expects an "r," or radius, attribute to be in the node data. To place the points of the arrows at the correct positions, I changed the refX and refY attributes so that the point of the arrow was at the edge of the node:

svg.append("defs").selectAll("marker")
    .data(["suit", "licensing", "resolved"])
  .enter().append("marker")
    .attr("id", function(d) { return d; })
    .attr("viewBox", "0 -5 10 10")
    .attr("refX", 10)
    .attr("refY", 0)
    .attr("markerWidth", 6)
    .attr("markerHeight", 6)
    .attr("orient", "auto")
  .append("path")
    .attr("d", "M0,-5L10,0L0,5");

Answer 2

This is really funny; I just solved this problem yesterday.

What I did is to end the path at the edge of the node, not at the centre. My case is more complicated because I use Bezier curves, not straight lines, but this might help you:

svg.append("svg:defs").selectAll("marker")
    .data(["default"])
  .enter().append("svg:marker")
    .attr("id", String)
    .attr("viewBox", "0 -3 6 6")
    .attr("refX", 5.0)
    .attr("refY", 0.0)
    .attr("markerWidth", 6)
    .attr("markerHeight", 6)
    .attr("orient", "auto")
  .append("svg:path")
    .attr("d", "M0,-2.0L5,0L0,2.0"); 


    links
      .attr("fill", "none")
      .attr("d", function(d) {
        var tightness = -3.0;
        if(d.type == "straight")
            tightness = 1000;

        // Places the control point for the Bezier on the bisection of the
        // segment between the source and target points, at a distance
        // equal to half the distance between the points.
        var dx = d.target.x - d.source.x;
        var dy = d.target.y - d.source.y;
        var dr = Math.sqrt(dx * dx + dy * dy);
        var qx = d.source.x + dx/2.0 - dy/tightness;
        var qy = d.source.y + dy/2.0 + dx/tightness;

        // Calculates the segment from the control point Q to the target
        // to use it as a direction to wich it will move "node_size" back
        // from the end point, to finish the edge aprox at the edge of the
        // node. Note there will be an angular error due to the segment not
        // having the same direction as the curve at that point.
        var dqx = d.target.x - qx;
        var dqy = d.target.y - qy;
        var qr = Math.sqrt(dqx * dqx + dqy * dqy);

        var offset = 1.1 * node_size(d.target);
        var tx = d.target.x - dqx/qr* offset;
        var ty = d.target.y - dqy/qr* offset;

        return "M" + d.source.x + "," + d.source.y + "Q"+ qx + "," + qy 
                + " " + tx + "," + ty;  // to "node_size" pixels before
                //+ " " + d.target.x + "," + d.target.y; // til target
      });

By the way; you'll have to do the same for the 'source' arrow head (I only have it at the target)

Answer 3

you may order the svg elements such that the circles will be rendered first, the lines with arrows thereafter (in d3 there is a .ordermethod, see here for details. for the record, the corrsponding part of the raphael api is discussed here).