Hm. You could just combine the two datasets into a single dataset. But that doesn't produce a very inspiring graph:

#! perl -slw
use strict;
use GD::Graph::bars;

my @d1 = (
    [ "2010/03/29 14:00:00", "2010/03/29 14:00:00", "2010/04/29 14:00:
+00", "2010/04/29 14:00:00" ],
    [ 13, 15, 20, 21 ],
);
my $g1 = GD::Graph::bars->new( 1000, 800 );
my $img1 = $g1->plot( \@d1 );

open PNG, '>:raw', "$0.png" or die $!;
print PNG $img1->png;
close PNG;
system "$0.png";
[download]

or maybe you need to retain two datasets and just combine the labels, to give this:

#! perl -slw
use strict;
use GD::Graph::bars;

my @d1 = (
    [ "2010/03/29 14:00:00", "2010/04/29 14:00:00" ],
    [ 13, 15 ],
    [ 20, 21 ],
);
my $g1 = GD::Graph::bars->new( 1000, 800 );
my $img1 = $g1->plot( \@d1 );

open PNG, '>:raw', "$0.png" or die $!;
print PNG $img1->png;
close PNG;
system "$0.png";
[download]

.

Or maybe I've combined the values the wrong way and what you want is this

#! perl -slw
use strict;
use GD::Graph::bars;

my @d1 = (
    [ "2010/03/29 14:00:00", "2010/04/29 14:00:00" ],
    [ 13, 20 ],
    [ 15, 21 ],
);
my $g1 = GD::Graph::bars->new( 1000, 800 );
my $img1 = $g1->plot( \@d1 );

open PNG, '>:raw', "$0.png" or die $!;
print PNG $img1->png;
close PNG;
system "$0.png";
[download]

And maybe you want a gap between the two this

#! perl -slw
use strict;
use GD::Graph::bars;

my @d1 = (
    [ "2010/03/29 14:00:00", undef, "2010/04/29 14:00:00" ],
    [ 13, undef, 20 ],
    [ 15, undef, 21 ],
);
my $g1 = GD::Graph::bars->new( 1000, 800 );
my $img1 = $g1->plot( \@d1 );

open PNG, '>:raw', "$0.png" or die $!;
print PNG $img1->png;
close PNG;
system "$0.png";
[download]