Learnability asks how easily a user can learn to use the product. A measure relates learning effort or successful first use to the users and tasks studied. Learning can be measured by time to competence, by the proportion of users who complete a task on the first attempt, or by the help needed to get there. Each is legitimate and they answer different questions. The population and the starting knowledge matter: an expert user learns a new product faster, and a study of experts says little about new users. The measure should name the task, the user group and the definition of learned that was used.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.