my $chart = Chart::Gnuplot->new(
    output => 'test.png', # not "terminal => 'windows'"
    ...

##</code><code>##

my $output_id = 0;  # put this before the MainLoop call

...

sub plot {
    $chart->plot3d($dataSet);
    my $newname = sprintf( "test_%03d.png", ++$output_id );
    rename( "test.png", $newname );

    my $top = $mw->Toplevel( -title => $newname );
    my $img = $top->Photo( -file => $newname );
    my $lbl => $top->Label( -image => $img )->pack;
}