问如何将d3图形保存为本地计算机上的图像？

EN

<div id="chart"></div>

<div id="canvas-option">
    <label>File Name : 
        <input type="text" id="canvas-filename" placeholder="image"/>.png</label>
        <input type="submit" value="Save">
</div>

 var canvas = d3.select("#chart").append("svg")
     .attr("id","svg_graph")
     .attr("width",width)
     .attr("height",height);



> canvas_option_form = $("canvas-option"),  canvas1 = $("chart"),
>   canvas_filename=$("canvas-filename")
>   canvas_option_form.addEventListener("submit",function(event)    {
>       event.preventDefault();         canvas1.toBlob(function(blob)       {
>           saveAs(blob,(canvas_filename.value ||
>                                 canvas_filename.placeholder)+".png");         },"image/png");     },false);