As per the details captured in issue #32 the javascript code portion of this Media Type has changed due to a platform/library change at Zabbix side.
A distinction is now made for the following Zabbix versions:

Zabbix <6.2 - use "zabbix.mailGraph.js"
Zabbix >=6.2 - <coming soon>

Please make sure you install the right javascript in your environment!