This page visualizes the activity of build slaves over time. For the date range specified (only YYYY-MM-DD dates are currently accepted), a visual timeline of events will be rendered for the selected group of slaves. Black represents a job executing on a slave. Whitespace means a job was not executing (the slave is idle, offline, etc). You ideally want to see solid black bars for as much of the graph as possible.
This service has hit it's database size quota. Try looking at data between 2013-08-15 and 2013-08-30 if current data is missing.
Source code lives here.