“The graph illustrates how much energy American consumed by 6 kinds of fuels from 1980 to 2002”
I wonder if there are any grammatical mistakes in the sentence, and if there are mistakes, please correct for me. THANKS!

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A straightforward alternative:

The graph shows American energy consumption from 6 fuel sources between 1980 and 2002.

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Hi

Americans (because you are talking about the people and not the country as a whole)

6 types of fuel (no s on fuel. 6 types shows is that there are many.)

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Don't worrier about mistakes

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It is a poor sentence because it is difficult to determine what word is modified by the phrase "by 6 kinds of fuel".  My best guess is that it is trying to be an adverbial phrase and that it is intended to modify the verb "illustrates".   I don't think the sentence can be easily repaired. It needs to be entirely rewritten.

"The graph breaks down by fuel type the quantity of energy Americans consumed between 1980 and 2002."