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?
First of all "barrels" is a unit of volume. For price, you need $200 per barrel. For some reason, "the price of oil" is a locution preferred over "the oil price."
Because you actually give the dates, you don't need to rely on the past perfect tense ("had plunged") to indicate the order of events. You may simply use the past tense to give a chronological narrative. In the same way, you don't need "before." It's clear that 2000 is before 2005. Thus I'd recommend:
The price of oil plunged to $200 per barrel in 2000 and then five years later surged to $600 per barrel.
