utilization.py: Enable stat tracking in middle of day
utilization-graph.py: Enable plotting of real-time data

- Fix bug where a computer is counted as idle when a user is still
  logged in (i.e. the session has a null end time)
- Add vertical line to indicate current time when plotting a day that
  is still in progress. This enables real-time plotting of lab
  usage
- Switch to a binomial-shaped weighted average