The moderator-only analytics for SE sites show already from where the traffic is coming from for a specific timeframe. This takes the form of a pie chart that shows how much traffic is direct, how much linked from other sites and how much from search engines.
Even more interesting would be to see the time evolution of the traffic sources. A steady growth is a requirement for graduating a site from beta, and the only way to achieve that is to get traffic from search engines. Referral traffic is pretty random, it's usually a short term boost but doesn't hold on very long.
I'd like to be able to graph not only the total number of visitors, but also just the visitors from one traffic source over time. Maybe even a stacked graph with all three traffic sources would make sense.
Here's a mockup of how I'd imagine the graph:
The advantage of splitting the traffic source would be that one could observe long-term trends in direct traffic and search engine traffic without trends being obscured by the far more volatile referral traffic.