I'm trying to understand something about the following sentence:
The graph highlights data about how much gas was produced in Russia between 2003 and 2008.
Would it be correct to include "that" or "which" between "gas was"?
If you asked the question:
How much gas was produced in Russia between 2003 and 2008?
Then you could answer: The graph highlights data about (followed by the question itself), right?
No, it would not be correct.
The problem arises because of the modifiers how much before gas.
Thus you can say:
The graph highlights data about the gas that was produced in Russia....
and
The graph highlights data about the gas, which/that was produced from Siberian coal, in Russia between....
But, if you ask the question:
How much gas was produced in Russia .....
The answer is:
The graph indicates how much gas was produced in Russia....
Equally you can say:
The graph indicates the gas that was produced in Russia....
But the conjunction that doesn't fit after how much
The graph highlights data about [how much gas was produced in Russia between 2003 and 2008].
No: you can't do that.
The bracketed element is a subordinate interrogative clause (embedded question) functioning as complement of the preposition "about". The meaning is:
"The graph highlights data about the answer to the question 'How much gas was produced in Russia between 2003 and 2008?'"
