Being able to monitor how quality evolves is a good start (see Time Machine).
The next step would be to manage your technical debt on your current developments in order to keep it under control.
To deal with that, Sonar offers you the Differential Services feature on measures, violations and coverage by unit tests on new code.
Differential views on Measures
To activate this service, pick the desired period in the drop down list Time changes...
When a period is selected, the differential values will appear next to the project measures:
Differential views on Violations
This service will help you focus your effort of quality on current developments:
Code Coverage on New/Changed Code
When activating the SCM Activity Plugin, you will be able to separate coverage by unit tests of new code from old (based upon a given cut-off date) in order to track the new code which does not have any/enough unit tests.
To see live examples, you can browse the following projects on Nemo:
Differential Views Settings
3 periods can be configured globally and will be available and common to all projects. Go to Configuration > General Settings > Differential Views to set these properties.
2 complementary periods can be configured and be specific to the current project. Select your project and go to Settings > Differential Views to set these properties