Your chart code (in the page) appears OK except for one major problem. You've embedded the whole <html>...</html> document inside your document.
<div class='post-body entry-content' id='post-body-2348845876793463601' itemprop='description articleBody'>
<html>
<head>
<!--Load the AJAX API-->
<script type="text/javascript" src="
https://www.gstatic.com/charts/loader.js"></script>
<script type="text/javascript">
// Load the Visualization API and the line package.
google.charts.load('current', {'packages':['line']});
...
</head>
<body>
<!--Div that will hold the linechart_material chart-->
<div id="linechart_material"></div>
</body>
</html>
<div style='clear: both;'></div>
So don't do that. A document should have only one head and one body. I don't know exactly what is happening, but apparently your document is confused. :)