/usr/bin/gfsplot is in gerris 20131206+dfsg-17.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 | #!/bin/sh
awk -v header="$1" -v plottmp=`mktemp` '
BEGIN {
  np = 0
  plot=""
  quote = sprintf("%c", 39)
  input = sprintf("%c-%c", 39, 39)
  print "set terminal wxt noraise"
}
{
  if ($1 == "#" && $2 == "1:x") {
    # Simulation plot
    if (plot) {
      # finish plot
      fflush(current)
      print plot
      print "! rm -f " current
      fflush()
    }
    else {
      # initial setup
      ns = 0
      for (i = 2; i < NF; i++) {
        split($i,a,":")
        number[ns] = a[1]
        symbol[ns++] = a[2]
      }
      for (i in symbol)
        gsub ("\\(" symbol[i] "\\)", "($" number[i] ")", header);
    }
    if (header) {
      # new simulation plot
      current = plottmp np++
      plot = header
      gsub (input, quote current quote, plot)
      gsub ("\\(t\\)", $(NF), plot)
    }
  }
  else if ($2 == "time:") {
    # timeseries plot
    print $0 >> plottmp
    fflush(plottmp)
    if (plot) {
      print plot
      fflush()
    }
    else if (header) {
      plot = header
      gsub (input, quote plottmp quote, plot)
    }
  }
  else if (plot)
    print $0 >> current;
}
END {
  if (current) {
    # finish simulation plot
    fflush (current)
    print plot
    print "! rm -f " current
  }
  print "! rm -f " plottmp
}' | gnuplot
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