Translate axes of 2D FFT plot to frequency

I am plotting the 2D fft of a matrix using mesh function.The matrix is a data from 16 linear sweeps of a FMCW wave with each sweep data arranged in a column( that is ,no of columns=16). NFFT along column =2^15 and NFFT along row = 256. fft2 is computed as fft2(x,2^15,256).From the figure,the x axis can be translated to frequency using fs/N(-N/2:N/2+1). How to translate the y axis to frequency?

[Adam]

Why can't you just do the same along both axes, using the relevant NFFT for each?