What is the difficulty curve?

The card at the bottom, "Difficulty curve", draws one bar per maze, in the order they print, as tall as the effort the simulated solver measured, colored by level. The levels step up, and inside each level the mazes go from the gentlest to the toughest. Point at a bar to read the maze's level, stars, cells, path, dead ends and effort; click it to see its page. On the left it counts the mazes of each level.