I liked Tim's graph of bloglines readers so I copied him.
I don't think this is so much a proxy for readership as it is a proxy for the growth of Bloglines itself. See below for a graph of my readers over the same time period; my general readership curve is pretty steady, nothing like the knee in the Bloglines graph.
I'm counting unique readers by IP address; not perfect, but close enough.
Python makes doing things like this such a joy. Here's my hack code. Ugly, but simple.
Update: the big jump in February Bloglines seems to be correlated with when I turned on full RSS.