By default, the tool proposes a "smoothing" (of the curve) of 3, which helps to profile the trends. On the nose flute Ngram, we can see a global increase of the occurences, with some peaks. Of course, the graphic includes the many native nose flutes that we can't substract easily from the results. However, the peaks stay significant.
On the right chart (1950-2000), there is a remarkable peak around 1977. I checked the books of this period, to understand what happened. The explanation wears a name : Roland Kirk, who died in 1977 at the top of his art and glory. As you can see, the peak took off from earlier.
Now, another interesting feature is the comparison with other expressions. I tried with nose flute, kazoo and slide whistle. They were invented in a 60 years timeframe: kazoo (1852), Nose flute (1892), Slide whistle (1910). Until 1940, the kazoo occurences graphic stays under the nose flute line, but then after, it blasts and reaches the sky! Why didn't nose flutes get such a success?