Using interp1 to write code

I am trying to complete steps 3-6 here. This is the code I have but all the graphs keep turning out the same...

clc
clear
HbData=dlmread('HbSpectra.txt');
IV = HbData(:,1);
FDV = HbData(:,2);
FM = 'o';
semilogy(IV, FDV, FM)
hold on
SDV = HbData(:,3);
SM = '+';
semilogy(IV, SDV, SM)
hold off

TT = linspace(400,896);

vl = interp1(IV,FDV,TT);
figure;plot(IV,FDV,'o',TT,vl)
title('linear interpolation')
vl = interp1(IV,SDV,TT);
hold on
plot(IV,SDV,'+',TT,vl)
title('linear interpolation')
hold off

vs = interp1(IV,FDV,TT,'spline');
figure;plot(IV,FDV,'o',TT,vs)
title('spline interpolation')
vs = interp1(IV,SDV,TT,'spline');
hold on
plot(IV,SDV,'+',TT,vs)
title('spline interpolation')
hold off

vh = interp1 (IV,FDV,TT,'pchip');
figure;plot (IV,FDV,'o',TT,vh)
title('pchip interpolation')
vh = interp1 (IV,SDV,TT,'pchip');
hold on
plot (IV,SDV,'+',TT,vh)
title('pchip interpolation')
hold off

Answer 1

[Star Strider]

It may only be possible to see the differences by subtracting them from each other or plotting them against each other.