How to plot the spectrum of a signal which is an array of amplitudes and not a sin or cos wave ?

I have an array,which is my signal. The x axis is not time,just the actual order of the values.Basically sample points.

How can I plot the spectrum of this signal(the amplitude against the frequency) ? I've tried numerous methods with the fft function,but I can't get it right,and I was wondering whether it is even possible,if the timespan is not specified. We only know the sampling frequency.

Signal_Spectrum = fft(Signal);

L = length(Signal);
f = 0:Frequency/2; 
P2 = abs(Signal_Spectrum/L);
P1 = 2*P2(1:L/2+1);
plot(f,P1)

I've tried this and I can't get it to work,because the vectors aren't the same length,so I am missing something. Also I want to store the spectrum on a variable,what should actually be named to be Signal_Spectrum ?