# transform data
 sub _log_scale {
        my ( $self, @y ) = @_;

        foreach my $y (@y) {
            foreach my $data (@{$y}) {
                if (!defined $data || $data <= 0) {
                    $data = undef;
                }
                else {
                    $data = log($data);
                }
            }
        }
        return @y;
    }

  # now "fix" the y labels
  sub _y_exp_format {
        my ( $v ) = @_;
        return 0 if !defined $v;
        return sprintf( "%.0f", exp($v));
    }

 if ($self->get_logscale) {
            $graph->set('y_number_format' =>          \&_y_exp_format,
                 # 'y_min_value'    => -1   );
        }