use strict;
use warnings;
use List::Util q(reduce);

sub xdi {
    my ($x, $d, $i) = @_;
    reduce{ $a*$b }map{ $_==$i || $d - $x->[$_] }keys @$x
}
sub lagc {
    my ($x, $f) = @_;
    [map{ $f->[$_] / xdi($x,$x->[$_],$_) }keys @$x]
}
sub lagx {
    my ($x, $L, $x0) = @_;
    reduce{ $a+$b }map{ $L->[$_] * xdi($x,$x0,$_) }keys @$x
}

##</code><code>##

use Chart::Gnuplot;
use List::Util q(reduce);

sub xdi {
    my ($x, $d, $i) = @_;
    reduce{ $a*$b }map{ $_==$i || $d - $x->[$_] }keys @$x
}
sub lagc {
    my ($x, $f) = @_;
    [map{ $f->[$_] / xdi($x,$x->[$_],$_) }keys @$x]
}
sub lagx {
    my ($x, $L, $x0) = @_;
    reduce{ $a+$b }map{ $L->[$_] * xdi($x,$x0,$_) }keys @$x
}

my $x = [-10 .. 10];
my $f = [map { $_/25 + sin $_ } @$x];
my $LC = lagc $x, $f;

my $xx = [map { $x->[0] + ($x->[-1] - $x->[0]) * $_/500 } -30 .. 530];
my $ff = [map { lagx $x, $LC, $_ } @$xx];

my $pts0 = Chart::Gnuplot::DataSet->new(xdata => $x, ydata => $f, style => "points");
my $pts = Chart::Gnuplot::DataSet->new(xdata => $xx, ydata => $ff, style => "lines");

Chart::Gnuplot->new(title => "f(x)", output => "plot.ps")->plot2d($pts0, $pts);