You follow a good structure :)
The line chart provides a breakdown regarding the growth of pupils graduated in Canada based on gender over a 15-year period between 1992 and 2007.
Actually , growth is an inference from the graph. In the introduction, simply say what the graph presents;The line graph provides information about male and female graduates in Canada from 1992 to 2007.
Overall, even though the number of female student graduated was always higher than the number of men, the increasing trends of both groups of students seems as likely to be similar. Interestingly, the gap between the number of women and men was wider and wider which female were 32% higher than man in the beginning and 52% at the end of period.
Overview should not contain data and details. They should move into the body paras(detailed paras) Here you should give the most obvious trend/ trends very briefly. For example;Overall, both male and female graduates have grown in their numbers and the number of female graduates have always been higher than male graduates throughout the period under review.