What i do is:
Create a Form with POST Method and a text area inside with style=”display:none;” to hidde it.
<form action="/GetBase64Chart/" method="post" name="formToSendBase64" id="formToSendBase64">
<textarea style="display:none;" id="txtareaBase64" name="txtareaBase64"> </textarea>
</form>
and with JS i create a button to “Export” the Base64 to the backend. Then i get the base64 of the canvas chart and i passed it to the textarea value like in the next code…
$("#exportButton").click(function () {
var canvas = $("#Chart .canvasjs-chart-canvas").get(0);
var dataURL = canvas.toDataURL();
document.getElementById("txtareaBase64").value = dataURL;
var form = document.getElementById("formToSendBase64");
form.submit();
});