bdr replication latency monitoring

I'm trying to follow the BDR monitoring docs:
https://wiki.postgresql.org/wiki/BDR_Monitoring

My postgres version string is (its from the 2nd Quadrant repo):
PostgreSQL 9.4.0 on x86_64-unknown-linux-gnu, compiled by gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3, 64-bit

My BDR plugin is version 0.8.0beta1

From the docs, I've come up with this query:
select slot_name, plugin, database, active, xmin,
pg_get_transaction_committime(xmin)
FROM pg_replication_slots ;

BDR is working. When I run that query, the 'xmin' value is always null, even though there is activity on the database. I do/can get a catalog_xmin value. Should I expect the 'xmin' value to be null? Is there another way to monitor the replication latency when using BDR?

Thanks,
Steve Boyle