In our IMPS/IMVS instruments the AC part of the stimulus has the form of
$$
\begin{equation}
P(t) = P\cos(\omega t).
\label{light}
\end{equation}
$$
Hence, any signal, say $I(t)$, can be written as:
$$
\begin{equation}
I(t) = X\cos(\omega t) + Y\sin(\omega t).
\label{i}
\end{equation}
$$
However, one may choose a different than $\cos(\omega t),\sin(\omega t)$ basis.
A popular basis choice is the pair: $ e^{i\omega t}$, and $e^{-i\omega t}$. It is convenient to use if one wants to separate positive and negative angular frequencies $\omega$. It is the case, for example, when a capacitor impedance $\frac{1}{i\omega C}$ is to be used.
To convert from the old to the new basis we write Eq. $ \ref{i}$ in the form:
$$
I(t) = X\left( \frac{e^{i\omega t} + e^{-i\omega t}}{2} \right) + Y\left( \frac{e^{i \omega t} - e^{-i\omega t}}{2i} \right)
$$
$$
I(t) = \frac{X-iY}{2} e^{i\omega t}+ \frac{X+iY}{2} e^{-i\omega t}
$$
So, the $I(t)$ components in the new basis are $\frac{X-iY}{2}$ and $\frac{X+iY}{2}$.
In the IMPS/IMVS literature often only the component at the positive frequency is considered i.e, $\frac{X-iY}{2}$.
Some authors normalize it (and you should not, see the post #5) with light intensity amplitude $P$ from Eq. $\ref{light}$. In order to use it, we need to find the proper component to the positive frequency
$$
P(t) = P\cos(\omega t) = \frac{P}{2}e^{i\omega t}+\frac{P}{2}e^{-i\omega t}
$$
To define the new quantity we take components at $e^{i\omega t}$ from the photocurrent $I(t)$ and the light intensity $P(t)$
$$
\begin{equation}
Q\equiv\frac{X-iY}{P}.
\label{q}
\end{equation}
$$
This can be depicted in a complex plot where $\Re(Q) = \frac{X}{P}$ and $\Im(Q)=-\frac{Y}{P}$.
One can see that it is straightforward to convert an X-Y plot into complex plot of $Q$.