These are sometimes called streamlines or streaklines, you can search for these words on the site. I adapted a classic algorithm (Whittaker, 1977) in https://observablehq.com/@visionscarto/deriving-pantelleria and https://observablehq.com/@fil/poisson-potential, but it's a little hidden (maybe I should extract it into its own separate notebook).

Thanks for pointing me in the right direction, I was indeed looking for something like this!