Let's look at the first two paragraphs for the question below.
The first chart below gives information about the money spent by British parents on their children’s sports between 2008 and 2014. The second chart shows the number of children who participated in three sports in Britain over the same time period.
Introduction and overview:
The line graphs show the average monthly amount that parents in Britain spent on their children’s sporting activities and the number of British children who took part in three different sports from 2008 to 2014.
It is clear that parents spent more money each year on their children’s participation in sports over the 6-year period. In terms of the number of children taking part, football was significantly more popular than athletics and swimming.
You just need one sentence for the introduction, and you can write it by paraphrasing the question statement (rewrite it in your own words). Try to write two sentences for your overview paragraph; in this case we can describe one main feature of each graph.