You have to use an array of arrays:

my @data = (
         ["1st","2nd","3rd","4th","5th","6th","7th", "8th", "9th"],
         [    1,    2,    5,    6,    3,  1.5,    1,     3,     4],
         [ sort { $a <=> $b } (1, 2, 5, 6, 3, 1.5, 1, 3, 4) ]
);
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The first array contains the X-values, and the rest of the arrays contain the Y-values to be plotted. After that, you do something like this: (untested)

# Create GD::Graph object
my $graph = GD::Graph->new(400, 300);

# Optionally set graph attributes
$graph->set(x_label => "X Label", y_label => "Y Label");

# Create the graph itself
my $gd = $graph->plot(\@data);

# And output the file on STDOUT (redirect to a file,
# or open a file yourself
print $gd->png;
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Most of this comes from the excellent GD::Graph documentation. Read it :-)

Arjen