I had some trouble with analog monthly stats not showing up for the last week and figured out that analog refuses to parse huge log files. I had one sitting at 3GB without being rotated and analog would error out with:
/usr/bin/analog: Warning F: Failed to open logfile
/var/log/httpd/access_log: ignoring it
After running gzip on the log file, analog was then able to produce the reports. I think I read somewhere that the limit may be 2GB but have not tested this.