index.html

## index.html
<!DOCTYPE html>
<html>
<head>
<script type="text/javascript" src="http://mbostock.github.com/d3/d3.js"></script>
<title>Download SVG</title>
<style type="text/css">
#download{
    cursor: pointer;
    text-decoration: none;
    color: black;
}
</style>
</head>
<body>

<div id="viz"></div>
<a href="#" id="download">Download</a>

<script type="text/javascript">

// Modified from https://groups.google.com/d/msg/d3-js/aQSWnEDFxIc/k0m0-q-3h1wJ

d3.select("#viz")
    .append("svg:svg")
    .attr("width", 300)
    .attr("height", 200)
    .style("background-color", "WhiteSmoke")
    .append("svg:rect")
    .attr("fill", "aliceblue")
    .attr("stroke", "cadetblue")
    .attr("width", 60)
    .attr("height", 40)
    .attr("x", 50)
    .attr("y", 50);

d3.select("#download")
    .on("mouseover", writeDownloadLink);

function writeDownloadLink(){
    var html = d3.select("svg")
        .attr("title", "svg_title")
        .attr("version", 1.1)
        .attr("xmlns", "http://www.w3.org/2000/svg")
        .node().parentNode.innerHTML;

    d3.select(this)
        .attr("href-lang", "image/svg+xml")
        .attr("href", "data:image/svg+xml;base64,\n" + btoa(html))
        .on("mousedown", function(){
            if(event.button != 2){
                d3.select(this)
                    .attr("href", null)
                    .html("Use right click");
            }
        })
        .on("mouseout", function(){
            d3.select(this)
                .html("Download");
        });
};

</script>

</body>
</html>