BTW, in a pinch since I'm already using Munin what I'm going to do is this :
- write a cronjob that fires maybe 2 to 4 times a day and does a "du -s" of directories I'm interested in , and stores the sizes in a file - write a simple munin plugin that reads the file (and munin will do the graphing for me)
Munin fires every 5 minutes so it will be a bit of waste since my cronjob will only fire a few times a day, but the weekly and monthly munin graphs will tell me what I want to know.