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まとめりゃいいじゃん、と思ったので、まとめた。
あー、そんなことより、課題やれ、って感じだよなぁ。

#! /usr/bin/perl
use strict;
use warnings;

my $usage = "usage: $0 data_filename [tail] [sleep_time]";

my $datafile = shift @ARGV or die "$usage\n";
my $tail = shift @ARGV || 10;
my @tail = ();
my $sleep_time = shift @ARGV || 0;

open DATA, "< $datafile" or die "can't open $datafile\n";
open GNUPLOT, "| gnuplot" or die "can't open gnuplot\n";
select( ( select(GNUPLOT), ( $| = 1 ) )[0] );

print GNUPLOT
    "set xrange [-1.6e11:1.6e11]\n",
    "set yrange [-1.6e11:1.6e11]\n",
    "set nokey\n";

while( <DATA> ){
    next if /^\#/;

    my( $x, $y ) = /^([\d.e+-]*)\t([\d.e+-]*)$/ or die;

    print GNUPLOT "plot '-' ps 2 pt 3, '-' ps 3 pt 5, '-' pt 2\n"; # '(0,0)`, `(x,y)`, `(x',y'),(x'',y``),,,'
    print GNUPLOT "0\t0\nend\n";
    print GNUPLOT "$x\t$y\nend\n";
    print GNUPLOT "0\t0\n" unless @tail;
    print GNUPLOT $_->[0], "\t", $_->[1], "\n" for @tail;
    print GNUPLOT "end\n";

    push @tail, [ $x, $y ];
    shift @tail if( @tail > $tail );

#    sleep $sleep_time;
    select undef, undef, undef, $sleep_time;
}

sleep 10;

close GNUPLOT;
close DATA;

sleepなメモ。
http://d.hatena.ne.jp/dasm/20041219#c
JEDのperl、v5.8.2だそうだけど、Time::HiResが入ってないっぽい。なんで?