You mean something like this:
close all
clc;
xi=0.3;
x=0:0.05:4;
d=1.-x.^2;
e=2*xi*x.^1;
fi=atand(e./d);
idx0 = find(x == 0);
idx1 = find(x == 1);
idx3 = find(x == 3);
plot(x(1:idx3),fi(1:idx3))
hold on
plot(x(idx3:end),fi(idx0:idx1),'r')
grid on
I didn't get your question properly. But as I understood you need just to play with indices.
