Part2: Please read part 1 first.

In 2009 has slightly it dropped slightly, but since 2010 it came has come back to 100 millions tonnes.

1/ "Has" is used in the present tense; it needs some connection with the present. 2009 was almost 10 years ago. It's completely in the past, so you use the past tense. "It dropped", not "It has dropped".

2/ Slightly is an adverb. (Most "-ly" verbs are in English, just as  "-mente" verbs are in Italian.) In English adverbs usually follow the verb that they describe; in this case the verb "dropped".

3/ When you say "Since", you are describing a period between some time in the past (in this case 2009) and now. "Came" is used in the context of the past; for example "It came back to 100 million tonnes in (note: not "since") 2009." As I said, "has" ties to the present, so when you say "it has come back" (as I have shown here) it means that it came back over a period starting in 2009, and ending now.

4/ "100 million" by itself would be understood, but it sounds a little incomplete without saying 100 million what.

The amount of European production is about 200 million tonnes. It also ezperienced experienced a little decrease around 2009, but then it mantained maintained pretty much at the same level.

You could say "It maintained the same level", or "It remained at the same level", but you can't mix them together.
The figure of China sharply increased, overtaking Europe in 2002 and reaching a peak of 800 millions of tonnes produced in 2014.

Part 1, because of Italki's ridiculous character limits.

Line Graph

1/ In English, unlike Italian, you generally capitalise the key words in a title.

2/ You always use "the" in front of either US or United States when you are using it as a noun (sostantivo). If you are using it as an adjective, you don't. Thus:

The agricultural production of the United States has remained constant for the last two years.

United States agricultural production has remained constant for the last two years.

Overall, the Chinese production showed a significant rise, while the American and European production fluctuated without significant changes.
The figure number of tonnes of crude steel tonnes produced by the US from 1999 to 2008 was around 100 millions.

1/ The word "figure" can be used to mean a "number", but it's not usually used as part of the description of a value. You would not say "the figure of something is something", though you might say something like "The figure for US steel production was 100 million tonnes".

2/ It would be uncommon to see "crude steel tonnes". Technically it's correct, but it makes "tonnes" the noun and "crude steel" an adjective describing the type of tonne. In this case the fact that it is crude steel is the more important piece of information, so that should be the noun.

3/ For a specific value, "million" is always singular. You would use the plural in an expression like "It will cost millions of dollars". However if it's specific, it will be "It will cost 100 million dollars".

To be continued.