FMCW SAR chirp return from target modelling
I am not sure you really understand the processing you do so I will try to explain what you measure.
When you use the returned chirp mixed against the reference chirp you get an instantaneous phase measurement. When you do the FFT the frequency domain represents distance - since the phase differences are changing faster as the distance is greater.
The inverse FFT of the Distance will give you (assuming high signal to noise ratio) a periodic signal that is the function of the distance you measured.
In the graph you show, it is not clear what you show since the phase is not presented.