Maybe it's true where you live, it is certainly not true in the English-speaking world. Fighting the Nazis was the primary way of describing the European war in contemporaneous popular and political writing.
Very interesting. In my reading of war-related English language literature from that period (Steinbeck, Orwell) I don't think I've encountered the term "Nazis", but that's hardly a representative sample.
Where I live (Poland), people referred to German occupiers as "Germans", and it is also how they spoke of themselves. For example, the sign on streetcarts said "only for Germans", not "only for Nazis" (https://en.wikipedia.org/wiki/Nur_f%C3%BCr_Deutsche).
Also, on your chart, you can see an over 50% drop of the usage of term "German" in 1945 (without any significant change for "Nazi"). It might have been caused by the US realization that the Western Germany will be an ally from now on, and change of rethoric.
At least here in Holland we tend to use 'Nazi' and 'German' interchangeably some of the time, and most of the time in the context of WW2.
The meaning of 'Germans' is much more context-dependent of course, to the point that I've noticed others and myself disambiguate by saying 'from Germany', because 'German' (Duitser) in itself has connotations (similar to jew/jood, or perhaps negroe/neger I guess).
Of course, being Dutch, many people will intentionally opt for the more rude version to get a rise out of people or 'be edgy'.
This is a reason of serious miscommunication when talking about this topic with the non-English native. I'm pretty sure English-speaking world doesn't mean fighting Slovakian Nazis, Latvian Nazis, or French Nazis, but those of two other very specific nationalities.
https://books.google.com/ngrams/graph?content=german%2C+nazi...