[NTG-context] Strange behavior with Metapost

Hello, Why curve drawn is not that expected ? Fabrice \starttext \startMPcode input hvdm; l:=2.8mm; r:=0.6; alfa:=45; beta:=0; gamma:=0; defineDefaultArrow (l, r, alfa, beta, gamma); numeric xmin, xmax, ymin, ymax; xmin :=-2; xmax := 4; ymax :=5 ;ymin:=-1; u := 1cm; pickup pencircle scaled 1pt ; drawarrow ((xmin,0) -- (xmax,0)) scaled u; drawarrow ((0,ymin) -- (0,ymax)) scaled u; draw tightfunction ("x", "2*(x-1)*(x-1)+2", xmin, xmax, (xmax-xmin)/10) scaled u; setbounds currentpicture to boundingbox (((xmin,ymin) -- (xmax,ymax)) scaled u); \stopMPcode \stoptext