We have a small high production office with about 8 drafters/designers, We have a hand written plot log on top of the plotter. Every plot sent is susposto be entered on the log. Number of sheets, project, date, and reason. We also have a slave driver in billing that keeps on us about this as she enters the info in to the clients billing statements, if they request it.
It's not the most accurate process as it sometimes dosn't get filled in, but it works quite well. If there are any simple programs, it might be an efficient route for us.