D3js force layout - line with gradient between nodes
<p>This is my d3 force layout: 
(Please run the code snippet)</p>

<p><div class="snippet" data-lang="js" data-hide="false" data-console="false" data-babel="false">
<div class="snippet-code">
<pre class="snippet-code-js lang-js prettyprint-override"><code>var width = 600,
    height = 600;

var svg = d3.select('body').append('svg')
    .attr('width', width)
    .attr('height', height);

var color = d3.scale.category20();

var dataNodes = [
    { x:   width/3, y:   height/3 , group: 0, color: 'blue'},
    { x: 2*width/3, y:   height/3, group: 1, color: 'red' },
    { x:   width/2, y: 2*height/3, group: 2, color: 'green'}
];

var dataLinks = [
  { source: 0, target: 1},
  { source: 1, target: 2},
  { source: 2, target: 0}
];

var force = d3.layout.force()
    .charge(-400)
    .linkDistance(height/2)
    .size([width, height])
    .linkStrength(1.3)
    .friction(0.8)
    .gravity(0.9);

force
    .nodes(dataNodes)
    .links(dataLinks)
    .start();

var link = svg.selectAll(".link")
      .data(dataLinks)
    .enter().append("line")
      .attr("class", "link");

var node = svg.selectAll(".node")
    .data(dataNodes)
  .enter().append("circle")
    .attr("class", function(d){ return "node " + d.color})
    .attr("r", width/20)
    .call(force.drag);

node.append("title")
      .text(function(d) { return d.color; });


force.on('tick', function() {
    link.attr("x1", function(d) { return d.source.x; })
        .attr("y1", function(d) { return d.source.y; })
        .attr("x2", function(d) { return d.target.x; })
        .attr("y2", function(d) { return d.target.y; });

    node.attr("cx", function(d) { return d.x; })
        .attr("cy", function(d) { return d.y; });
});</code></pre>
<pre class="snippet-code-css lang-css prettyprint-override"><code>.node {
    fill: #ccc;
    stroke: #fff;
    stroke-width: 0;
}
.node.blue {
    fill: blue;
}
.node.red {
    fill: red;
}
.node.green {
    fill: green;
}

.link {
    fill: none;         
    stroke: black;
    stroke-width: 20px;
}</code></pre>
<pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.11/d3.min.js"&gt;&lt;/script&gt;</code></pre>
</div>
</div>
</p>

<p>This is what i want to achieve:
<a href="https://i.stack.imgur.com/kgt8l.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/kgt8l.png" alt="enter image description here"></a></p>

<p>How is this possible ? How can i apply a gradient on the links between the nodes ?
If something is not clear please ask.</p>

<p>Thanks !</p>