Mgstat produces html reports for years, months and days (with png or 
gif charts generated with GD library) that shows you how many bytes you 
saved by using this excellent free Apache module. It produces history 
files so you can rotate your logs without losing previous statistics.  
You can also run mgstat on the same log file and statistics won't be 
doubled because program will start from the place where previous 
processing ended. You can also run mgstat on totally unsorted (date order) 
log file (that were produced for example by merging various log files); 
program will run a little bit slower but will produce correct reports. 

WWW: http://wizard.ae.krakow.pl/~mike/mgstat/