Statistical Information

In the past few days I tried to organize what statistical information can and should be displayed, and in what form. Once an item is returned the Centrifuge system creates a record in the logs table with the item id, checkout date and checkin date - no member information concerning the item is retained. Therefore, it is technically possible to find out which items get checked out most often, what the average duration of checkout is and so on. Furthermore we have current numbers of items checked out. Especially the latter two seem to have only limited usefulness.
The length of checkout can be easily skewed if everybody brings their books back within one month - but one person returns 6 after a year. Just waiting for more data to let the outlier drop in importance also doesn't work since the data cannot be too old (say 30 or 90 days) to even be relevant, still.
I'm uncertain if I'm going to include that statistic anywhere but surely not on the main page. I'll probably make a graph under 'Tools' to show lines of checkin/out over time.
I'm also uncertain about leaving the absolute number of checked out items on the main page, since it can be misinterpreted. Yes, this number should not rise infinitely but sometimes a lot of new members join and start reading and that should not be alarming.
I'm thinking of splitting that number into *absolute checkouts* and *checkouts overdue* (aka 30 days or more).
It was fun playing with the graphing possibilities of the PEAR engine and I'd like to use a few more graphs, I just don't know where yet...Suggestions?