I want to graph the picture in other axis by ezplot

this is my code

//////////////////////// clear all

syms l n_eff

 a=700*10^-9
    k=2*pi/l
    %n1=sellmeier_glass(l)
    B1=1.03961212
    B2=0.231792344
    B3=1.01046945
    C1=6.00669867*(10^(-3))*10^(-12)
    C2=2.00179144*(10^(-2))*10^(-12)
    C3=1.03560653*(10^(2))*10^(-12)
    n1 = sqrt(1+(B1*(l.^2)./((l.^2)-C1))+(B2*(l.^2)./((l.^2)-C2))+(B3*(l.^2)./((l.^2)-C3)))

    u= a*k*(((n1^2)-(n_eff)^2))^(1/2)
    w= a*k*(((n_eff)^2)-1)^(1/2)

 J0 = besselj(0,u)

 J1 = besselj(1,u)

 K0 = besselk(0,w)

 K1 = besselk(1,w)

 f = J0/(u*J1) - K0/(w*K1)

 ezplot(f,[300*10^-9,2000*10^-9,1,2])

////////////////////////////

this ezplot 'f' graph show me n_eff, l-axis graph of course.

however i want to make N, l-axis 'f' graph

N is the new variable : N = n_eff - l*(dn_eff/dl)

so i make dn_eff/dl = -diff(f,l)/diff(f,n_eff)

(maybe it is true mathematically...)

Help me T.T