I found in an English learners’ textbook a sentence which goes, “Many people helped me on my journey.” That’s no problem. That makes sense to me. What about “in my journey?” Both seem to be okay. However, Google Ngram Viewer says “in my journey” is used a lot more often (link). Then comes up a question. This extract is from a English textbook for Japanese high school students at a lower intermediate or beginner level. Then why “on my journey” rather than “in my journey?” I wonder if “on” is preferred in this context?
Concerning prevalence of use, you made a little mistake in Ngram: You looked for "in my journey, on my journeys". Using only the singular, we see a quite different usage curve.
More to the core of the question:
The noun 'journey' is also used metaphorically. "In my journey to become a better programmer, many people helped me."
Here, 'on' is slightly off.
On the other hand, if 'journey' refers to an actual, physical journey, at least in the singular 'on' will be the better choice, as it (loosely) describes an event, even if that event may stretch over a longer period of time.