Hi Marcus,
Y-Axis is the amplitude "abs(of the complex signal)".
X-Axis is not pure frequency domain or time domain, because the figure is drown from follow:
I generate ZC sequence "its amplitude equals to 1 in frequency domain" then I made IFFT and transmit the signal using USRP. The environment is a cable. I received the signal from another antenna of the same USRP. In the receiver I made FFT and plot the correspond figure. Since, I am the source file in the transmitter transmit the signal many times"repeat activated". The last figure which I sent it is the signal in the frequency domain which are repeated with each transmission "I put them all together to compare them."
I attached the same figure with more explanation "each black block is the signal in the frequency domain", but block 1 .....n is the same signal transmitted in different times.