Without them, the meaning of the graph would be obscured, and it would read like a collection of disjointed statements. You start with a hypothesis. Either it's clear in the beginning, or it develops during the course of your investigation. You try to prove it or disprove it. You risk ending up with no story.
Story is usually controversial.
Your standard of evidence is high. Never rely on only one source. Story often involves corruption, incompetence, bribery, hypocrisy. Writing techniques for investigative story. Use people to tell the story.