The problem
Most retrospective formats tell you what happened this sprint. The happiness radar tells you how the team is doing across time, are they more or less satisfied with their work, their tools, their collaboration, their growth?
The radar format is a spider chart on a whiteboard: axes for Team, Process, Tooling, Product, Learning, and whatever else matters. Each person rates each dimension on a simple scale. The team average creates the radar shape.
The chart looks great on a whiteboard. But a whiteboard chart is useless for trend analysis, you need to compare this sprint's shape to last sprint's. That means documentation, and nobody does it consistently unless the process is nearly effortless.