Culprit

https://github.com/jenkinsci/warnings-ng-plugin/blob/e659c7ada2792573a475e50242ef4c9d9bdb9b85/plugin/src/main/resources/io/jenkins/plugins/analysis/core/portlets/IssuesChartPortlet/portlet.jelly#L18-L24

Solution

Use st:bind with var attribute to define a global variable, which can then be accessed from JavaScript. Use data attributes with e.g. span element to pass whatever other data is needed.
(No docs exist AFAICT? Perhaps we could improve here )