Nice guesses so far; CTG in particular is hot on the trail (his interpretation of both plots is correct). But there are some more clues
is there a description of main algorithms and data structures of stunts? (i know about some work on track elements but car and car collision - also the reason of PG (overflowing/round presicion of floats - but some line of pseudocode would be nice ))
There is no neat documentation that I know of, though the assets of the restunts project include a substantial amount of unsystematically annotated assembly. (Writing notes on some of that stuff, specially on track building, is in my TODO list for almost three years now...)
btw - that is strange - y is axe of height?
Yup, y is height (cf. 3D coordinates in CarBlaster and stressed). Naturally, There are y values along the x and z ones; I didn't log them thus far due to minor technical annoyances.