Unfortunately, there is no option nodgrid3d
(like there is nosurface
) to turn off the gridding for individual plot parts. I think you must first plot the gridded data to a temporary file and plot this instead:
set dgrid3d 30,30,3
set table 'map.grid'
splot 'map.dat' w l
unset table
unset dgrid3d
set pm3d
splot 'map.grid' w l, 'path.dat' w l lt -1