D3 diagonal using coordinates instead of objects

I am looking for a way to create a line like the one you get using d3.svg.diagonal but instead of using source and target objects, use coordinates. I tried fiddling with the projection function but no success.

Here is a fiddle I found with it being done between objects: http://jsfiddle.net/bmdhacks/qsEbd/5/

Short version of my objective:

Lets say I have two points, like this picture:

straight line

I want to be able to have them do the following by only providing coordinates for start and ending:

curved line, same points

whats in the fiddle, if it helps:

var data = [ {name: "p1", children: [{name: "c1"}, {name: "c2"}, {name: "c3"}, {name: "c4"}]}];
var width = 400, height = 200, radius = 10, gap = 50;

// test layout
var nodes = [];
var links = [];
data.forEach(function(d, i) {
    d.x = width/4;
    d.y = height/2;
    d.children.forEach(function(c, i) {
        c.x = 3*width/4;
        c.y = gap * (i +1) -2*radius;

        var a = {x:c.y, y:c.x};
        var b = {x:d.y, y:d.x};
        links.push({source: b, target: a});

var color = d3.scale.category20();

var svg = d3.select("#chart").append("svg")
        .attr("width", width)
        .attr("height", height)
var diagonal = d3.svg.diagonal()
        .projection(function(d) { 
            return [d.y, d.x]; });

var link = svg.selectAll(".link")
        .attr("class", "link")
        .attr("d", diagonal);

var circle = svg.selectAll(".circle")
        .attr("class", "circle");

var el = circle.append("circle")
        .attr("cx", function(d) {return d.x})
        .attr("cy", function(d) {return d.y})
        .attr("r", radius)
        .style("fill", function(d) {return color(d.name)})
        .append("title").text(function(d) {return d.name});


Ok, so I decoded an SVG file from illustrator and figured out the values to make that happen. Here it is for reference of others. This example only works horizontally, but I am sure you can figure out the vertical version... :)

d=[{x1:100,y1:100,x2:300,y2:500}, ... ]

var CustomLine = function(d,i){
        var x1 = d.x1;
        var y1 = d.y1;
        var x2 = d.x2;
        var y2 = d.y2;
        var s = Math.abs(x1-x2)*0.70; // strength 
        return "M"+x1+","+y1+" C"+(x1+s)+","+y1+" "+(x2-s)+","+y2+" "+x2+","+y2;