HTML documents using the Highcharts library version
7.x (released on 2018-12-11) or above are not rendered correctly in API v1. In order to resolve the issue, you have these two options:
1/ Use the 6.2 version which is known to work with API v1
<script src="http://code.highcharts.com/6.2/highcharts.js"></script>
<script src="http://code.highcharts.com/6.2/highcharts-more.js"></script>
2/ Or switch your implementation to
API v2 which supports the latest Highcharts library version.