Rules Compliance Index (RCI) Plugin for SonarQube 
Requires SonarQube 5.6+ (tested against 5.6, 5.6.3 (LTS), 6.0, 6.1, 6.2, 6.3)
This plugin calculates a metric based on the weighted value of issues and the number of lines of code (default settings are given in parentheses and can be configured on a global and per project level; other values are taken from the core metrics):
Issue weight = blocker violations * weight (10)
+ critical violations * weight (5)
+ major violations * weight (3)
+ minor violations * weight (1)
+ info violations * weight (0)
Rules Compliance Index = max(1.0 - (Issue Weigth / Lines of Code) * 100, 0)
The plugin also adds a (configurable) Rules Compliance Rating metric to visualize the RCI (score an A-rating with a rules compliance index of 97%).
Installation
Install the plugin via the Update Center in the SonarQube administration pages. Or to install the plugin manually; copy the .jar file from the release to the extensions/plugins directory of your SonarQube installation.