A lagometer is a display of network latency on an Internet connection and of rendering by the client. Lagometers are commonly found in computer games or IRC where timing plays a large role. Quake and derived games commonly have them.
Advanced lagometer consists of two lines - bottom and top. The bottom line advances one pixel per each snapshot received from server (by default they are being sent at 20 snapshots per second rate), while the top one advances one pixel per each frame that is rendered by client. Thus, if the machine framerate was 20 per second, both lines - top and bottom - would run at the same speed.
More info at Wikipedia, earthli.
The upper graph advances one pixel for every rendered frame on the client side.\