Looks like you may have to wait till the script file is loaded
$(document).ready(function () {
if ($("#amchartsLib").length == 0) {
var script = document.createElement("script");
script.type = "text/javascript";
script.src = "/Scripts/amcharts/amcharts.js";
script.id = "amchartsLib";
document.body.appendChild(script);
script.onload = function () {
DrawChart();
}
}
})