Here are my answers to some questions about writing task 1:
1) Can I start my introduction like this: "It is on a line graph playing out over a 60 year time period that one sees..."?
No. That sentence looks very strange to me as a native speaker, and it won't impress the examiner. Just stick to the simple approach that I recommend in my task 1 lessons here on the website.
2) What will happen if my overview paragraph is different from what the examiner would write for an overview?
I tell my students to choose two main or general points for their overview (summary) paragraph. There are usually more than two main points to choose from, so don't worry about whether the examiner would choose something different. If your overview sentences summarise the information well, you'll be fine.
3) Is writing task 1 less important than task 2?
Yes. Task 1 is worth one third of your writing score. Task 2 is worth two thirds.
4) Can I write more than 150 words?
Yes. There is no maximum word count.
5) Is every word counted, even words like "a"?
Yes. All words are counted, even small words like "a" and "of".