Extra effort is the first outcome the MLQ measures: followers going beyond what they first meant to do, and frequently beyond what they believed they could do, the defining effect of transformational leadership in the authors' account (moving people from doing a task for pay to doing it because they identify with it and take pride in it). The authors report that since the 1980s research has shown that transformational leadership outperforms transactional leadership at producing extra effort, commitment and satisfaction.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.