edit
seq 10 | awk '{printf("%d %f\n", $0, $0 * $0 + rand());}' > data gnuplot
f(x)=a*x*x+b*x+c a=0.5;b=0;c=0; fit f(x) 'data' u 1:2 via a,b,c plot f(x), 'data' using 1:2