Thanks for your answers, I could significantly reduce my long timeline code.
1. Variable names x and y are special and cleared before every graphic. This is related to the fact that “z" is defined as a shortcut for "(x,y)”. Both of these are traditional.
Every metapost graphic does at least this at its start:
save x,y; currentpicture := nullpicture; currentpen:=defaultpen;
so pair xx[]; would have worked just fine.
In general, I would advice to stay away from single-letter variable names inside MPinclusions, and use CamelCase to avoid clashes with metafun internals.
So, got one more z-related error in Metapost. I bet this is absolutely the n.1 on help-lists. Btw, if I were a scientist, I'd realized that defining x as a pair is quite heretic… And that sounds much familiar to an historian. Damien Thiriet