I know we use had when we want to tell of events in the past — e.g., The train had left before I reached the station. I want to write about an up-and-down trend in a line graph at different points. Assume I used simple past tense in the first two sentences in a paragraph. Now, in the third sentence I write:
The oil price had plunged to 200 barrels in 2000 before it surged to 600 barrels in 2005.
Should I write this without had? Is it optional?