took a nosedive by 5 tonnes and afterwards showed a moderate decrease to (Probably confusing but "of" sounds more correct.)

roughly 6 tonnes. Thus, ("Thus" is synonymous with "Therefore", Sweden and Portugal having an equal number of emissions did not happen because of the previously mentioned nosedive. I would recommend saying a more neutral connective like "Furthermore".)

in 2007 the number of emissions in Sweden and Portugal was equal.
To sum up, it can be concluded that such countries ("countries such") as

Portugal and Spain depicted a ceiling tendency. Conversely, the UK represented ("represented" doesn't sound right here, "displayed" is more apt) a descending trend as well as Sweden after having had a definite period of vacillations. (lol, had to google this one, at least I learned something.)

fin--------------------------------

I was really nit-picky to be fair, but I had to be since your English is already good.

Also, I'm not a teacher at all, just a native.

The (I would use "This" as it directly refers to the graph attached. "The" is more vague. Not a big deal really though.)

line graph outlines the approximate number of the emissions of CO2 per person in 4 European countries over the period from ("of" sounds right here. I don't know why but "over the period from" sounds disjointed. You could also avoid this entirely by removing "over the period" and it makes perfect but more simplistic sense: "...CO2 per person in 4 European countries from 1967")

1967 to 2007.

A glance at the graph reveals that the (More proper.)

countries with the lowest level of CO2 emissions at the onset, i.e. Portugal and Italy, sharply surged their numbers. (?) (I could be wrong but I'd say it's safer to say "had their numbers surge sharply". The word surge usually implies result and not cause so I find it strange when used to describe an action taken.)

For instance, Portugal accounting for less than 2 metric tonnes in 1967, reached approximately 6 tonnes in (You could use "within" instead of "in" to sound more fancy, though I don't think that's your goal.)

40 years. Turning back to (I'm probably being picky here as this is technically correct, but other phrases that could be used here are "When it comes to Italy..." or "Additionally, Italy". It's strange (to me) to start the sentence that way when you have so recently mentioned Italy in a previous sentence. I would personally change this particular use of it and keep the (just look for the pink highlighting) later use of it.)

Italy, the number of emissions in this country, starting from just over 4 tonnes, virtually doubled its figures over the given period. Whilst these two countries demonstrated a ceiling trend (Is this a jargon term?)

, the UK with the biggest proportion of emissions of 11 tonnes, sequentially ("subsequently" is correct, as the word "sequentially" implies it follows a logical pattern - which you could argue is true - but "subsequently" is the word I think you want to use since it means "after the (previously mentioned event)". Not to be confused with "consequently" which is used when the previously mentioned event directly caused a result: e.g. "as a consequence".)

plunged its figures by roughly 3 tonnes.

Turning to Sweden, it is crystal clear that its ("their", also if you were to use "it's" remember the apostrophe: "it's" is short for "it is". The sentence "Turning to Sweden, it is crystal clear that it is numbers were fluctuating" should sound incorrect at your good level of English.)

numbers were fluctuating during the whole time. (Again, can't say why but this sounds wrong. You could simply say "throughout" instead: "...their numbers were fluctuating throughout".)

Demonstrating quite high indexes of emissions in 1967, it was sharply rocketing ("their numbers sharply rocketed" "it" is too vague.)

and reached an all-time high of almost 11 tonnes in 10 years. However, over the next decade, it ("Sweden's numbers" again, keep it relative to the direct pro-nouns. You could say "their numbers" again but using different words where possible is advised for high-quality English.)