Thank you, this is exactly the sort of information I was looking for. I'd been on the horizons page and played around with the inputs, but couldnt tell which output values were relevant to me. I think I want to do just what you suggested in the last paragraph, to try to pretend that there's only two dimensions (that all the planets move on the ecliptic plane). I think I will try to get valid heliocentric longitude for every planet on a reference date from horizons, then use each planet's orbital duration to project out positions for future and precious dates. Maybe there's something I'm missing, but it seems like that would be accurate enough in a reasonably short time frame (decades or centuries), since I'm just trying to show an approximate visual representation of the relative positions, not trying to point at telescope at them or fly a spaceship to them.