How to draw nullclines based on phase portrait

Hi

I'm creating a phase portrait by running a loop with different initial conditions to get trajectories and show the stability of the different fixed points. I want to draw nullclines to the system but I don't have the differential equation. I only have x,y,dx,dy

How can I plot it? Something like this https://controls.engin.umich.edu/wiki/index.php/File:Nullclines.jpg I don't have the equation so I don't think I can use pplane function.

Thank you Janna