create a Matlab plot with two y axes, then add a third and fourth line to it

plotyy.m

x = -pi:.1:pi;
f1 = sin(x);
f2 = 100*cos(x);
f3 = cos(x/2);
f4 = 100*sin(x/2);

[axes,linehandle1,linehandle2] = plotyy(x, f1, x, f2);
linehandle3=line(x,f3,'Parent',axes(1),'LineWidth',2,'LineStyle','--');
linehandle4=line(x,f4,'Parent',axes(2),'LineWidth',2,'LineStyle','--');