$chart->plot2d($val1) if $val == 1;
$chart->plot2d($val1,$val2) if $val == 2;
$chart->plot2d($val1,$val2,$val3) if $val == 3;
etc...

##</code><code>##

my @datasets;
for my $i (...) {
    ....
    push @datasets, Chart::Gnuplot::DataSet->new(...);
    $chart->plot2d(@datasets);
}