This example shows how to add to d3.js node-link graphs support for any number of multiple parallel links between two nodes. A fuller post on the involved calculations can be found here.
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<html>
<head>
<link href="style.css" rel="stylesheet">
</head>
<body>
<div class="example-controls">
<label for="radio-exact">Exact</label>
<input type="radio" name="calculation" id="radio-exact" value="e" onchange="radioChange(this)">
<label for="radio-approx">Approximate</label>
<input type="radio" name="calculation" id="radio-approx" value="a" onchange="radioChange(this)">
</div>
<div id="example1" class="example-view"></div>
<!-- scripts -->
<script src="https://d3js.org/d3.v3.min.js"></script>
<script src="ParallelLinksExample.js"></script>
<script src="main.js"></script>
</body>
</html>
https://d3js.org/d3.v3.min.js