Plot the magnitude of coefficients against frequencies of DCT transform When we perform DCT transform using matlab:

 number_of_sample_data = 100;
 f=0.1; %frequency
 fs=1; %sampling rate 
 phi_0=0
 t=0:1/fs:((number_of_sample_data-1)*1/fs); %generate 100 samples
 x=sin(2*pi*f*t+phi_0);
dct_transform=dct(x); %perform discrete cosine transform
figure(1), stem(dct_transform);


this piece of code will plot the magnitude of coefficients against frequencies or something else?
Actuually, that code doesn't work without defining axis t variable, and frequency f, regarding to what that code does, is simple:

x=sin(2*pi*f*t);        %x is a sinusoidal vector of frequency f and length t;
dct_transform=dct(x);   %dct_transform is a vector containing the unitary discrete transform of x
figure(1), plot(dct_transform); %plots the the unitary discrete transform of the sinusoid against default axis in Matlab


So you are just plotting the discrete transform vector against sample position, wish is what Matlab does by default.