If you're using Oracle and a Backend that aren't set to a GMT timezone, this may cause problems in displaying metrics. In this case, you'll need to set up the Tomcat server to be in GMT.
Add "--catalina-opts -Duser.timezone=GMT" to the end of the takipi-server.sh script command line.
./takipi-server.sh start --catalina-opts -Duser.timezone=GMT
This will fix the problem and metrics will start showing up for new data (however, this won't fix the issue for older data).