[Bioperl-l] about the gc% plotting

[Magic Fang]

What the $seq must be? Does it needs a feature descripts the gc% when i want to plot gc%?
I just got an axis of gc% figure, no gc% line. And then my codes are: 
$generic=Bio::SeqFeature::Generic->new(-start=>1,
 -end=>$seq->length,
 -seq_id=>$seq->display_name);
$generic->attach_seq($seq);
......
$panel->add_track($generic,
 -glyph=>'dna',
 -height=>100,
 -fgcolor=>blue,
 -axis_color=>purple,
 -bgcolor=>green,
 -linewidth=>3,
 -outlinecolor=>red);
Can anybody tell me why? And another thing is if we can not use Bio::Graphics::Glyph::dna etc. directly?
Magic Fang