HTML2Canvas converting overflowed content to image

I have a div which is pretty overflowed. It basically includes a big organization chart. What I want to do is exporting whole content of div rather than visible part with html2canvas library but I couldn't achieve it so far. Following piece of code doesn't render full content. Is there a way to achieve it?

function export(){  
    html2canvas( [ document.getElementById('diagram') ], {
        onrendered: function(canvas) {

            var dataUrl = canvas.toDataURL();
            window.open(dataUrl, "toDataURL() image", "width=800, height=800");
            //Canvas2Image.saveAsPNG(canvas);
        }
     });
}

I am using BasicPrimitives library to generate organization charts. It takes a div and insert all elements to it. Since my chart is moderately big, it overflows from its container. Xhtml code is as follows:

<rich:panel style="float: left; width: 100%;">
     <div style="float: left; height:600px; margin-left: 1%;  width: 19%; border-style: dotted; border-width:1px;">
          Some irrelevant content
     </div>
     <div id="diagram" class='diagram' style="float: right; height:600px;  width: 59%; border-style: dotted; border-width:1px;">
          This is the div all charts are dynamically inserted
     </div>
     <div style="float: left; height:600px; margin-left: 1%;  width: 19%; border-style: dotted; border-width:1px;">
          Some more irrelevant content 
     </div>
</rich:panel>