Tried another direction and this works.

#!/usr/bin/perl -w


use LWP::Simple;
use Fcntl qw(:DEFAULT :flock);
my $interval = 60; # seconds between calls
my $url='http://www.cryptonator.com/api/ticker/btc-gbp';
my $gpdata;
$SIG{TERM}=sub{close(PIPE);if(defined(*OUT)){close(OUT);print "done\n"
+;exit;}};
sub init{
    local $/=undef;
    $gpdata=<DATA>;
    open PIPE, "|gnuplot -persist -geometry 512x384+0+0" || die "Can't
+ initialize gnuplot\n";
    select((select(PIPE), $| = 1)[0]);
}
sub fetch{
     my $replie = get($url);
    print"$replie \n";
    
    
    
    my($a,$b,$c,$d)= $replie  =~ /^.+?(\d+\.\d+).+?(\d+\.*\d*).+?(\d+\
+.\d+).+?(\d+).*$/;
    $a="not defined" unless defined($a);
    $b="not defined" unless defined($b);
    $c="not defined" unless defined($c);
    $d="not defined" unless defined($d);
    open(OUT,">>","/home/philip/Desktop/PERLBITCOIN/TICKER/bitgbpZ.dat
+") or die "BIT File open error $!\n";
    print OUT "$d,$a,$c,$b\n";
    close(OUT);
    
}
sub update{
    print PIPE $gpdata;

}

sub loop{
    fetch();
    sleep 1;
    update();
}

sub main{
    init();
    do{
        loop;
        sleep 60;
    }while(1)
}
main;

__DATA__
reset
set terminal 'wxt'
delta_v(x) = ( vD = x - old_v, old_v = x, vD)
old_v = NaN
unset key
set xtics border in scale 0,0 nomirror rotate by -90  offset character
+ 0,0  autojustify
set datafile separator ','
set bmargin at screen 0.2
set palette defined (-1 "red", 1 "#1B6821")
set cbrange [-1:1]
set grid
set yrange [*:*]
unset colorbox
set style fill solid noborder
#set xrange [1446160802-946684800:1446160902-946684800]
set xdata time
set timefmt "%s"
set xtics 60
set format x "%H:%M:%S" # or whatever
set xlabel "time"
#set xtics autofreq
data=system("tail -n 1 /home/philip/Desktop/PERLBITCOIN/TICKER/bitgbpZ
+.dat |  cut -d ',' -f2")
set title data
plot '< tail -n 60 /home/philip/Desktop/PERLBITCOIN/TICKER/bitgbpZ.dat
+'  using 1:($2):((delta_v($2) > 0.0) ? 1 : -1 ) title data with lines
+p lt 6 lw 2 palette
[download]

Thanks to all for help and suggestions