I investigated:
if the chart button is displayed the html head code is
<script src="/media/com_hikashop/js/hikashop.js?v=160" type="text/javascript"></script>
<script src="/media/system/js/mootools-core.js" type="text/javascript"></script>
<script src="/media/system/js/core.js" type="text/javascript"></script>
<script src="/media/system/js/mootools-more.js" type="text/javascript"></script>
<script src="/media/com_hikashop/js/vote.js" type="text/javascript"></script>
if the chart button is hidden is
<script src="/media/com_hikashop/js/hikashop.js?v=160" type="text/javascript"></script>
<script src="/media/com_hikashop/js/vote.js" type="text/javascript"></script>
<script src="/media/system/js/mootools-core.js" type="text/javascript"></script>
<script src="/media/system/js/core.js" type="text/javascript"></script>
<script src="/media/system/js/mootools-more.js" type="text/javascript"></script>
The mootool scripts are loaded after vote.js, this causes an error