You're right that QtCharts needs QApplication, I hadn't realized that. I see 2 options:
1. Change ign-gui to use QApplication and test all ign-gazebo plugins to verify things still work. This change breaks ABI and would need to go into the upcoming Ignition release, Dome.
2. If the above doesn't work, we won't be able to use QtCharts and will need to look for another approach.