f(x) = cos(x)
set logscale
derr(x) = -sin(x)
G(h,x) = abs( ( f(x+h)−f(x) )/h - derr(x) )
B(h,x) = abs( ( f(x+h) − f(x−h) )/(2*h) - derr(x) )
D(h,x) = abs( ( −f(x+2*h)+8f(x+h)−8f(x−h)+f(x−2*h) )/( 12*h) - derr(x) )