set view equal xyz
  set ticslevel 0
  set colors classic
  set view 90,0,2.5,1

  set xlabel 'x'
  set ylabel 'y'
  set zlabel 'z'

  file = 'grid.txt'
  splot \
    NaN w p lt 7 pt 5 title 'boundary', \
    NaN w p lt 1 pt 5 title 'fluid', \
    file u ($1==0?$2:NaN):3:4 w d lt 7 notitle, \
    file u ($1>0 ?$2:NaN):3:4 w d lt 1 notitle

  pause -1 'Press Enter to continue: '