var doctype = '<?xml version="1.0" standalone="no"?><!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd" [<!ENTITY nbsp " ">]>'; 
function convertToPng(src, w, h) { 
      var canvas = document.createElement('canvas'); 
      var context = canvas.getContext('2d'); 
      canvas.width = w; 
      canvas.height = h; 
      context.drawImage(src, 0, 0); 
      var png; 
      try { 
        png = canvas.toDataURL("image/png"); 
      } catch (e) { 
        if ((typeof SecurityError !== 'undefined' && e instanceof SecurityError) || e.name == "SecurityError") { 
          console.error("Rendered SVG images cannot be downloaded in this browser."); 
          return; 
        } else { 
          throw e; 
        } 
      } 
      return png; 
    } 
    function isElement(obj) { 
      return obj instanceof HTMLElement || obj instanceof SVGElement; 
    } 
    function reEncode(data) { 
      data = encodeURIComponent(data); 
      data = data.replace(/%([0-9A-F]{2})/g, function (match, p1) { 
        var c = String.fromCharCode('0x' + p1); 
        return c === '%' ? '%25' : c; 
      }); 
      return decodeURIComponent(data); 
    } 
    function uriToBlob(uri) { 
      var byteString = window.atob(uri.split(',')[1]); 
      var mimeString = uri.split(',')[0].split(':')[1].split(';')[0] 
      var buffer = new ArrayBuffer(byteString.length); 
      var intArray = new Uint8Array(buffer); 
      for (var i = 0; i < byteString.length; i++) { 
        intArray[i] = byteString.charCodeAt(i); 
      } 
      return new Blob([buffer], {type: mimeString}); 
    } 
    function downLoad(el, name) { 
      if (!isElement(el)) { 
        throw new Error('an HTMLElement or SVGElement is required; got ' + el); 
      } 
      if (!name) { 
        console.error("文件名為空!"); 
        return; 
      } 
      var xmlns = "http://www.w3.org/2000/xmlns/"; 
      var clone = el.cloneNode(true); 
      clone.setAttribute("version", "1.1"); 
      if (!clone.getAttribute('xmlns')) { 
        clone.setAttributeNS(xmlns, "xmlns", "http://www.w3.org/2000/svg"); 
      } 
      if (!clone.getAttribute('xmlns:xlink')) { 
        clone.setAttributeNS(xmlns, "xmlns:xlink", "http://www.w3.org/1999/xlink"); 
      } 
      var svg = clone.outerHTML; 
      var uri = 'data:image/svg+xml;base64,' + window.btoa(reEncode(doctype + svg)); 
      var image = new Image(); 
      image.onload = function () { 
        var png = convertToPng(image, image.width, image.height); 
        var saveLink = document.createElement('a'); 
        var downloadSupported = 'download' in saveLink; 
        if (downloadSupported) { 
          saveLink.download = name + ".png"; 
          saveLink.style.display = 'none'; 
          document.body.appendChild(saveLink); 
          try { 
            var blob = uriToBlob(png); 
            var url = URL.createObjectURL(blob); 
            saveLink.href = url; 
            saveLink.onclick = function () { 
              requestAnimationFrame(function () { 
                URL.revokeObjectURL(url); 
              }) 
            }; 
          } catch (e) { 
            saveLink.href = uri; 
          } 
          saveLink.click(); 
          document.body.removeChild(saveLink); 
        } 
      }; 
      image.src = uri; 
    } 

<div id="svg-parent-div"><svg>...</svg></div> 

var el = d3.select("#svg-parent-div").node().children[0]; 
download(el,"下載圖片"); 

el.setAttributeNS(xmlns, "xmlns", "http://www.w3.org/2000/svg"); 
el.setAttributeNS(xmlns, "xmlns:xlink", "http://www.w3.org/1999/xlink");