proc drawarrow {start end mol} {
  set middle [vecadd $start [vecscale 0.85 [vecsub $end $start]]]
  graphics $mol cylinder $start $middle radius 0.30 filled yes
  graphics $mol cone $middle $end radius 0.50
}

proc porcplot {mol graphicmol} {
  set firstsel [atomselect $mol all frame 0]
  set lastsel [atomselect $mol all frame [molinfo $mol get numframes]]
  
  foreach startcoord [$firstsel get {x y z}] lastcoord [$lastsel get {x y z}] {
    puts "$startcoord $lastcoord"
    drawarrow $startcoord $lastcoord $graphicmol
  }

  $firstsel delete
  $lastsel delete
}