You can do this by setting two_axes. I have found letting GD::Graph set the range of the Y axis works better than manually setting them. You can label the Y axis by setting the y1_label and y2_label values.

use strict;
use GD::Graph::lines;

my @data = ( [1, 2, 3, 4, 5, 6, 7, 8, 9,],
         [ 8, 8.25, 8.5, 8.35, 8.76, 8.21, 8.6, 8.8, 8.5],
         [ 57.4, 57.6, 57.8, 57.9, 57.4, 58.5, 57, 57.87, 58.34] );

my $my_graph = new GD::Graph::lines (600, 400);

$my_graph->set(
    title => "My Graph",
    x_label => 'X axis label',
    y1_label => 'Y1 axis label',
    y2_label => 'Y2 axis label',
    boxclr => '#C0C0C0',
    two_axes => 1,
);

$my_graph->set_legend("data set 1", "data set 2");

my $gd = $my_graph->plot(\@data) or die $my_graph->error;
open(IMG, '>file.gif') or die $!;
binmode IMG;
print IMG $gd->gif;
close IMG;
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