Export Graph to PNG?

[GravisTechRyan]

I've been beating my head against this for a little bit. Are there any examples of how to export the graph to a PNG? I've been trying to post imgURI to a script that writes it out to a png but its always just a blank image.

Javascript

   var dataURL = VWCchart.dataURI().then(({ imgURI, blob }) => {

    var url = 'post_chart.php';//your url to the server side file that will receive the data.
    var http = new XMLHttpRequest();
    http.open('POST', url, true);
    var data = imgURI;

    http.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');

    http.onreadystatechange = function() {
        if(http.readyState == 4 && http.status == 200) {
            //console.log(data);
        }
    };


    http.send('img=' + data);
   });

PHP post_chart.php

<?php

if($_POST) {
   $data = $_POST['img'];
   $data = str_replace('data:image/png;base64,', '', $data);

    $f = fopen('weekly_report/test.png', 'w+');
    fwrite($f, base64_decode($data));
    fclose($f);
}