Couln't find exactly what I was looking for, so I decided to write my own Nucleus plugin. And here it is.
This plugin records all unique visitors (based on time/ip) to the database. It shows a graphical display of the number of visitors, and some simple statistics. This is a first release (0.9) and any feedback is welcome.

features:

• bot/crawler detection
• country detection
• configurable colors
• up to 14 different imagesets

to do:

• color picker
• fancy icons for the statistics
• images for graphs
• per-blog statistics
• date format configurable through admin panel

installation:
upload, and insert in your skin where you want the counter to be displayed (typically in the sidemenu)

download here:

NP_DBUCounter.zip (0.9)