gistfile1.txt

<button type="button" class="btn btn-default" id="save">Save Image</button>
<script>
var a = document.createElement("a");

d3.select("#save").on("click", function() {
  var canvas = document.querySelector("canvas"),
      context = canvas.getContext("2d");

  context.clearRect(0, 0, canvas.width, canvas.height);

  var data = d3.select("svg")
        .attr("version", 1.1)
        .attr("xmlns", "http://www.w3.org/2000/svg")
        .node().parentNode.innerHTML;

  var DOMURL = window.URL || window.webkitURL || window;

  var image = new Image;
  var svg = new Blob([data], {type: 'image/svg+xml;charset=utf-8'});
  var url = DOMURL.createObjectURL(svg);

  image.onload = function() {
      context.drawImage(image, 0, 0);
      DOMURL.revokeObjectURL(url);

      var canvasdata = canvas.toDataURL("image/png");

      var pngimg = '<img src="'+canvasdata+'">';
      d3.select("#pngdataurl").data(pngimg);

      a.download = "sample.png";
      a.href = canvasdata;
      a.click();
  };
  image.src = url;

});
</script>