Change d3 force layout link style to match d3-tree look - d3.js

Question

I am undertaking yet another attempt to draw a family tree with d3. For this, I would like to use the usual node-link graph (like this one):
But with a link style like that usually found in d3 trees, i.e. be the Bezier curves with horizontal (or vertical) ends:
Is it possible to change the links accordingly, without diving into the d3-force code?

Answer 1

If you are just looking to match the style of the links, no need to dive into the d3-force code, it only calculates position, not anything related to styling.
Each link has a x and y values for both the source and the target. If you replace the line that is found linking source and target in most force layout examples with a path, you can use these x and y values to pretty much style any type of link you want.
I'm using d3v4+ below - your examples use d3v3.
Option 1 - Use the Built In Links
In d3v3 you would use d3.svg.diagonal, but now there is d3.linkVertical() and d3.linkHorizontal() to achieve the same thing. With this we can use:
d3.linkVertical()
.x(function(d) { return d.x; })
.y(function(d) { return d.y; }));
And then shape paths representing links with:
link.attr("d",d3.linkVertical()
.x(function(d) { return d.x; })
.y(function(d) { return d.y; }));
I've only done a vertical styling below - but you could determine if the difference in the x coordinates is greater than the y coordinates to determine if you should apply horizontal or vertical styling.
var svg = d3.select("svg");
var nodes = "abcdefg".split("").map(function(d) {
return {name:d};
})
var links = "bcdef".split("").map(function(d) {
return {target:"a", source:d}
})
links.push({target:"d", source:"b"},{target:"d", source:"g"})
var simulation = d3.forceSimulation()
.force("link", d3.forceLink().id(function(d) { return d.name; }))
.force("charge", d3.forceManyBody().strength(-1000))
.force("center", d3.forceCenter(250,150));
var node = svg.append("g")
.selectAll("circle")
.data(nodes)
.enter().append("circle")
.attr("r", 5)
var link = svg.append("g")
.selectAll("path")
.data(links)
.enter().append("path")
simulation
.nodes(nodes)
.on("tick", ticked)
.force("link")
.links(links);
function ticked() {
link.attr("d", d3.linkVertical()
.x(function(d) { return d.x; })
.y(function(d) { return d.y; }));
node
.attr("cx", function(d) { return d.x; })
.attr("cy", function(d) { return d.y; });
}
path {
stroke: black;
stroke-width: 2px;
fill:none;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/5.7.0/d3.min.js"></script>
<svg width="500" height="300">
Option 2 - Manually Specify Path
We can substitute the line used to connect nodes with a path, we can supply the d attribute of the path manually given the datum of the path contains the x,y of the target and the source. Perhaps something like:
path.attr("d", function(d) {
var x0 = d.source.x;
var y0 = d.source.y;
var x1 = d.target.x;
var y1 = d.target.y;
var xcontrol = x1 * 0.5 + x0 * 0.5;
return ["M",x0,y0,"C",xcontrol,y0,xcontrol,y1,x1,y1].join(" ");
})
Again, I've only done only one styling here, this time horizontal, but adding a check to see if horizontal or vertical links are needed should be fairly straightforward:
var svg = d3.select("svg");
var nodes = "abcdefg".split("").map(function(d) {
return {name:d};
})
var links = "bcdef".split("").map(function(d) {
return {target:"a", source:d}
})
links.push({target:"d", source:"b"},{target:"d", source:"g"})
var simulation = d3.forceSimulation()
.force("link", d3.forceLink().id(function(d) { return d.name; }))
.force("charge", d3.forceManyBody().strength(-1000))
.force("center", d3.forceCenter(250,150));
var node = svg.append("g")
.selectAll("circle")
.data(nodes)
.enter().append("circle")
.attr("r", 5)
var link = svg.append("g")
.selectAll("path")
.data(links)
.enter().append("path")
simulation
.nodes(nodes)
.on("tick", ticked)
.force("link")
.links(links);
function ticked() {
link.attr("d", function(d) {
var x0 = d.source.x;
var y0 = d.source.y;
var x1 = d.target.x;
var y1 = d.target.y;
var xcontrol = x1 * 0.5 + x0 * 0.5;
return ["M",x0,y0,"C",xcontrol,y0,xcontrol,y1,x1,y1].join(" ");
})
node
.attr("cx", function(d) { return d.x; })
.attr("cy", function(d) { return d.y; });
}
path {
stroke: black;
stroke-width: 2px;
fill:none;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/5.7.0/d3.min.js"></script>
<svg width="500" height="300">
Option 3 - Use a Custom Curve Generator
I include this because I just recently answered a question on custom curves, that by chance uses the same styling. This way we can define the path of each link with:
var line = d3.line().curve(d3.someCurve))
and
link.attr("d", function(d) {
return line([[d.source.x,d.source.y],[d.target.x,d.target.y]]);
})
I've added a couple lines to build on the example above too, the curves can be either vertical or horizontal:
var curve = function(context) {
var custom = d3.curveLinear(context);
custom._context = context;
custom.point = function(x,y) {
x = +x, y = +y;
switch (this._point) {
case 0: this._point = 1;
this._line ? this._context.lineTo(x, y) : this._context.moveTo(x, y);
this.x0 = x; this.y0 = y;
break;
case 1: this._point = 2;
default:
if (Math.abs(this.x0 - x) > Math.abs(this.y0 - y)) {
var x1 = this.x0 * 0.5 + x * 0.5;
this._context.bezierCurveTo(x1,this.y0,x1,y,x,y);
}
else {
var y1 = this.y0 * 0.5 + y * 0.5;
this._context.bezierCurveTo(this.x0,y1,x,y1,x,y);
}
this.x0 = x; this.y0 = y;
break;
}
}
return custom;
}
var svg = d3.select("svg");
var line = d3.line()
.curve(curve);
var nodes = "abcdefg".split("").map(function(d) {
return {name:d};
})
var links = "bcdef".split("").map(function(d) {
return {target:"a", source:d}
})
links.push({target:"d", source:"b"},{target:"d", source:"g"})
var simulation = d3.forceSimulation()
.force("link", d3.forceLink().id(function(d) { return d.name; }))
.force("charge", d3.forceManyBody().strength(-1000))
.force("center", d3.forceCenter(250,150));
var node = svg.append("g")
.selectAll("circle")
.data(nodes)
.enter().append("circle")
.attr("r", 5)
var link = svg.append("g")
.selectAll("path")
.data(links)
.enter().append("path")
simulation
.nodes(nodes)
.on("tick", ticked)
.force("link")
.links(links);
function ticked() {
link.
attr("d", function(d) {
return line([[d.source.x,d.source.y],[d.target.x,d.target.y]]);
})
node
.attr("cx", function(d) { return d.x; })
.attr("cy", function(d) { return d.y; });
}
path {
stroke: black;
stroke-width: 2px;
fill:none;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/5.7.0/d3.min.js"></script>
<svg width="500" height="300">
This option will work with canvas as well (as will option 1 if I'm not mistaken).