This scatter plot shows the relationship between complexity and coverage for each class in the project, similar to that provided by Clover.
Why is this useful? This gives us ideas to explore. Ideally, we want to increase test coverage and reduce complexity. So put your attention on the classes towards the top and left of the chart which are more complex and less covered, reduce their complexity and / or increase their coverage, and "push" them towards the bottom right corner where classes are simpler and better covered.
Hover over a bubble to see details about the class or classes at that point.