I don't know if there's a straightforward option in html2canvas to do this (i.e. an option to set all overflow to visible) but a roundabout way might be to set the parent of the diagram element's overflow property to visible when your export function is called, then set it back to hidden again on html2canvas' onrendered callback so that the user has minimal time to perceive it:
function export(){
document.getElementById('diagram').parentNode.style.overflow = 'visible'; //might need to do this to grandparent nodes as well, possibly.
html2canvas( [ document.getElementById('diagram') ], {
onrendered: function(canvas) {
document.getElementById('diagram').parentNode.style.overflow = 'hidden';
var dataUrl = canvas.toDataURL();
window.open(dataUrl, "toDataURL() image", "width=800, height=800");
//Canvas2Image.saveAsPNG(canvas);
}
});
}