The graph shows us the teacher's performances in a period of five years, and it is obvious that each of them had performed differently, and reacted differently to certain conditions, and a big factor is also the fact that they are all new employees.
Jill; Started as an average teacher, and obviously needed the longest time of all to adapt to the new job, so remained average teacher for three years, but after that starts to improve, and by the end of the fifth year, is the best teacher of the five.
Ed; Started very badly, as the worst teacher, but in the second year makes huge improvement, and than his performances go down and up every year. Obviously, is inconsistent, and probably lacks motivation at times.
Ralph; Thrived after the initial pressure in the first year, and was the best teacher, but seems to become very leaned back afterwards, so had a huge drop in performances in the second year, and dropped even further to the bottom in the next years.
Tina; Started pretty good, but had a slight drop in the second year. However, bounced back to the initial performances, and remained constant.
Sam; Started very badly, but than slightly improved, to only drop to the bottom in the third year, and than improve again to a level of average teacher. Also, pretty inconsistent.
It seems that Jill and Tina are the best suited for the job out of the five new teachers in the school.