set terminal postscript color
set xlabel "log h"

set output "errorplot.eps"
set title "Error of difference quotient"
set ylabel "error"
plot "dat.txt" u 1:2 t "f1" w lp, "dat.txt" u 1:3 t "f2" w lp, "dat.txt" u 1:4 t "f3" w lp