There may be another way to do this, but when using XML::Generator it appears you have to build the XML from the inside out (or 'bottom up') like this:

use XML::Generator;

my $xml = new XML::Generator;
my @list;

while (<DATA>) {
  chomp;
  my ($m, $d) = split(' ', $_);
  push(@list, $xml->set( { label => $m, value => $d }));
}
print $xml->chart({ caption => 'Chart Title', subcaption => 'Subtitle'
+ }, @list);
__END__
January 17400
February 19800
March 21800
April 23800
May 29600
June 27600
July 31800
August 39700
September 37800
October 21900
November 32900
December 39800
[download]

Also, I'm not sure why you are mixing XML::Simple (i.e. XMLout) and XML::Generator. I would use just one or the other.