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.
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Differential Views are available since Sonar 2.5
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