Modifiability asks how much effort it takes to change the product without introducing defects. A measure relates change effort or change failure to the changes made. Modifiability is measured after the fact, from real changes. Two useful shapes are the effort per change and the proportion of changes that caused a defect. The second is often more persuasive, because it captures the hidden cost of a fragile area. The measure should segment by area, since an average across a healthy codebase and a fragile one hides the area that will cost the next quarter.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.