in gnuplot, do something like:
plot [x=-pi:pi]
x,2*sin(x),2*sin(x)-sin(2*x),2*sin(x)-sin(2*x)+(2./3.)*sin(3*x),2*sin(x)-sin(2*x)+(2./3.)*sin(3*x)-(1./2.)*sin(4*x)+(2./5.)*sin(5*x)-(1./3.)*sin(6*x)+(2./7.)*sin(7*x)-(2./8.)*sin(8*x)+(2./9.)*sin(9*x)-(2./10.)*sin(10*x)+(2./11.)*sin(11*x)

...and look at ac-ap notes for how to include this
(this is a Fourier polynomial for x)