You have heard of the concept RTFM. I suggest you do. It is all *very* well documented. Here is the FM. Here is a bit of sample code to give you the idea:

sub graph_by_user {
    my ( $png, $data, $title ) = @_;
    require GD;
    require GD::Graph::hbars;
    my $X   = 600;                              # x size
    my $Y   = 60+ (15 * scalar keys %$data);    # y size
    my $GD = GD::Graph::hbars->new( $X, $Y );
    my ( @users, @categorised, @uncategorised );
    for my $user ( sort keys %$data ) {
        push @users, $user;
        push @uncategorised, $data->{$user}->[0];
        push @categorised, $data->{$user}->[1];
    }
    $data = GD::Graph::Data->new([ \@users, \@uncategorised, \@categor
+ised ]) 
        or die $GD->error;
    my @atribs = (
        y_label     => "Total Reviews",
        long_ticks  => 1,
        cumulate => 1,
        title   => $title,
        dclrs   => [ qw[ blue green ] ],
    );
    $GD->set(@atribs);
    $GD->set_legend( 'Uncategorised', 'Categorised' );

    my $gd = $GD->plot( $data ) or die $GD->error;
    open my $img, ">$png" or die "Couldn't write $png PNG $!";
    binmode $img;
    print $img $gd->png or die "Error printing png: $!:";
    close $img;
}
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x_labels_vertical => 1 is the attribute you want to make the labels vertical.