Leadership is an identifiable, observable set of skills and practices available to anyone, not a trait reserved for a few charismatic people at the top; personal-best cases come from every level, sector and country. The role models people name are local (a family member, a teacher or coach, and for adults the immediate supervisor), so every manager is the most important leader to their direct reports and is accountable for the example they set whether or not they intend it. Leadership makes a measurable difference: people report using on average about a third of their talent under their worst leader and nearly all of it under their best, and the frequency of the Five Practices explains far more of direct reports' engagement than their own demographics or organizational context. The most lasting test of a leader is how far they develop leadership in others.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.