Select/Group by/Order by question - Mailing list pgsql-general

I'm trying to create a summary log by hour.  Here's the query (somewhat
simplified):

   select  to_char(mtrantime,'mm-dd hh AM') as datetime,
   count(*) as tot from memtran
   group by datetime
   order by datetime;

The problem is this produces the data in the following order:

 datetime   | tot
-------------+-----
04-08 01 PM |  14
04-08 02 PM |  15
04-08 03 PM |  23
04-08 07 AM |   8
04-08 08 AM |  54
04-08 09 AM |  30
04-08 10 AM |  11
04-08 11 AM |  10
04-08 11 PM |   7
04-08 12 PM |  10

What I'd really like is to get it in chronological order by hour:

04-08 07 AM |   8
04-08 08 AM |  54
04-08 09 AM |  30
04-08 10 AM |  11
04-08 11 AM |  10
04-08 12 PM |  10
04-08 01 PM |  14
04-08 02 PM |  15
04-08 03 PM |  23
04-08 11 PM |   7

I would prefer not to show the time of day in 24 hour format, but
there doesn't appear to be a way to order by something that
isn't in the select and group by clause and I don't want to display
the hour twice.

Putting the AM/PM before the HH (which looks a bit clumsy) almost works,
except that 12PM gets sorted to the bottom after 11PM.

Is there an easy way around this?
--
Mike Nolan