Ggtcf

I looked at sample http://bl.ocks.org/mbostock/raw/4063570/:

It produces nice merged lines from source target from left to right.

In my case I need to layout nodes manually and put x, y coordinates. In this case the lines are not merged at source nodes. Here is the test code that reproduce this problem:

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;
 nodes.push(d);
 d.children.forEach(function(c, i) 
 c.x = 3*width/4;
 c.y = gap * (i +1) -2*radius;
 nodes.push(c);
 links.push(source: d, target: c);
 )
)

var color = d3.scale.category20();

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

var link = svg.selectAll(".link")
 .data(links)
 .enter().append("path")
 .attr("class", "link")
 .attr("d", diagonal);

var circle = svg.selectAll(".circle")
 .data(nodes)
 .enter()
 .append("g")
 .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);

There is sample of this at http://jsfiddle.net/zmagdum/qsEbd/:

However, it looks like the behavior of curves close to nodes are opposite of desired. I would like them to start straight horizontally at the nodes and make a curve in the middle. Is there a trick to do this?

This solution is based on excellent @bmdhacks solution, however, I believe mine is slightly better, since it doesn't require swapping x and y within data itself.

The idea is that you can use diagonal.source() and diagonal.target() to swap x and y:

var diagonal = d3.svg.diagonal()
 .source(function(d) return "x":d.source.y, "y":d.source.x; ) 
 .target(function(d) return "x":d.target.y, "y":d.target.x; )
 .projection(function(d) return [d.y, d.x]; );

All x y swapping is now encapsulated within the code above.

The result:

Here is also jsfiddle.

Note that in the blocks example, the x and y values are swapped in the links. This would normally draw the links in the wrong place, but he's also supplied a projection function that swaps them back.

var diagonal = d3.svg.diagonal()
 .projection(function(d) return [d.y, d.x]; );

Here's your jsfiddle with this technique applied:
http://jsfiddle.net/bmdhacks/qsEbd/5/