This refers to the link where it clearly writes that:
The FrFT argument $u$ is neither a spatial one $x$ nor a frequency $\xi$. We will see why it can be interpreted as linear combination of both coordinates $(x,\xi)$
But I did not see the relation between the domain of Fractional Fourier transform ($u$) and space and frequency $(x,\xi)$. I can understand that domain '$u$' is a linear combination of '$(x,\xi)$' but my question what is the relation? Can anyone point out the exact relation?