You will quite often get to see IELTS Line graphs in Writing task 1 along with bar charts, pie charts, tables, maps, diagrams etc. However, you should focus on the right aspects for a high score.
Firstly, your introduction must be stated very clearly and try to include all the information, in addition you should put all main trends in an overview statement which comprise one paragraph.
Secondly, avoid making spelling errors and you must incorporate a high range of vocabulary for an outstanding answer.
You must structure your paragraphs in a logical way so that it is easier for the examiner to read and mark you accordingly. Hence, it will also save the examiner's time.
Also, you are advised to not spend more than 20 minutes on Task 1. Last but not the least, aim to write 160-190 words.