The green line is the plot of
f(x) = 6100000/x, or, to put it another way, the plot of
x × y = 6100000. The knee in the curve ought to be where
x = y = √6100000 ≈ 2470. But look at the chart! Just by eyeballing it, we can see that 2470 is nowhere near what anyone would call the knee.