Chirp Signal in MATLAB

%Code:-

Fs = 8000;

t = 0:1/Fs:4;

f = 400;

fi =[];

temp=0;

for i=1:length(t)

    fi=[fi temp];

    temp=temp+0.000025*f;

end

y = sin (2 * pi * t.*fi );

plot(y);

sound (y, Fs);