p 'decay_euler_rk4.dat' u 1:2 w l, '' u 1:3 w l , '' u 1:4 w l
feuler(x)=a*x+b
frk4(x)=m*x+q
fit feuler(x) 'decay_euler_rk4_errors.dat' u (-log($1)):(log($2)) via a,b
fit frk4(x) 'decay_euler_rk4_errors.dat' u (-log($1)):(log($3)) via m,q
p 'decay_euler_rk4_errors.dat' u (-log($1)):(log($2)) w lp title "Euler",'' u (-log($1)):(log($3)) w lp title "RK4", feuler(x), frk4(x)