Azzi is correct in his characterization of the default x-axis. The values that you are getting with your code:
A = abs(fft(thesignal));
plot(A);
Are magnitudes associated with frequencies of the form (2*pi*k)/N radians/sample (or k/N cycles/sample) where N is the length of the input signal, and k = 0,1,2,...N-1
Note that the first value is associated with k=0 which corresponds to a frequency of 0 radians/sample, or DC.
Also, keep in mind that discrete frequencies only increase in oscillation up to a point, and then actually start to decrease.
For an even length, N, at k=N/2, the radian frequency is pi radians/sample. Clearly this has a period of 2 samples.
However at k = 3N/4 (provided that is an integer), the radian frequency is 3pi/2 radians/sample and that has a period of 4 samples.
For a real-valued signal, you can choose to plot the output of fft() (the DFT) for only 1/2 the values so you can obtain an easier to interpret plot where the frequency actually increases monotonically.
You can also use fftshift, if you prefer to visualize 0 frequency at the center of the output.
Ex:
n = 0:159;
x = cos(pi/4*n); % pi/4 radians/sample
xdft = fft(x);
N = length(x)
k = 0:N-1;
plot(k,abs(xdft))
Note that peaks at k=20 and k=140 (which is N-k). k=20 is equal to a radian/frequency of (2*pi*k)/N or pi/4 radians/sample (1 cycle/8 samples)
