x=[1 4 7 12 14 21]
y=[12.5 13.2 15.7 16.1 19.5 20.7]
p=polyfit(x,y,1)
T=polyval(p,x)
e=y-T;
re=max(abs(e));
plot(x,y,'*',x,T,'r',x,T+min(e),'s-',x,T+max(e),'k',[x(e==re) x(e==re)],[T(e==re)+re T(e==re)],'m'])