The binman chronicles
Player Valuation: £80m
All your chart underlines is that Djokovic has been more consistent in his career with less plateauing periods than Nadal and Federer. He doesn't dramatically "take off" after the others lose form.
Nadal
He plateaued during his peak years because the other two were better.
Nadal pounding his body into the ground meant pretty much he was Federer's age body wise.
Murray and Del Potro being made of cheesestrings too also plumped Djokovic's stats as they weren't around much when they should have been up there challenging. It's all there in black and white.