I have modified the GatewayMetricsAssertion Policy that is included with the Tactical GatewayMetricsAssertion package.
This gwmetricschartjs policy now displays some gatewaymetrics as a chart.js graph.
URL parameters are used to display the metrics avgFrontendResponseTime, avgBackendResponseTime, successfulRequestRate, policyViolationRate, routingFailureRate. Of course you can modify it in the code (XSLT).
Test the policy with a webbrowser with internet access by a get request to the policy.