my $computator = Computator->new($p, $mw, whatever ...);
$computator->reset();
my $plot = $mw->Button(-text=>'Plot', -command=>sub { $computator->init()} )
    ->pack(-side=>'left');

# create a stop-button
my $stop = $mw->Button(-text=>'Stop', -command=>sub { $computator->stop()})
    ->pack(-side=>'left');