How to plot dy/dx versus x of a given y=f(x) plot in LTspice?
Bad news, you can't plot a derivative of a variable with respect to another derivative in LT spice. You can however plot the derivative with respect to time like this \$ \frac{dx}{dt}\$ or \$ \int{x}dt\$. 

You can plot integrals with respect to time like listed here
There is also a way to integrate a signal by holding down control in the time domain.

I was able to get a graph by inserting d(V(vo)) on the horizontal axis and d(V(vi)) (my signals are not the same as yours) however, I'm not sure that \$ {\frac{dx}{dt}}/{\frac{dy}{dt}}\$ is going to get you the same as \$ \frac{dx}{dy} \$

If not you can always export into python, excel or matlab and use the math functions there.