Hi,
As PSTricks (which I prefer) is not yet well integrated, I decided to learn
Metapost starting with a simple code.

\starttext
\startMPcode
numeric xmin, xmax, ymin, ymax;
xmin := -2.5; xmax := 2.5; ymax :=2.5 ;ymin:=-2.5; u := 2cm;
vardef f(expr x) = -x*x+1 enddef;
xinc := 0.001;
path pts_f;
pts_f := (xmin,f(xmin))*u
for x=xmin+xinc step xinc until xmax+xinc:
.. (x,f(x))*u
endfor;
path axe_x,axe_y;
path screen;
screen=(xmin,ymin)*u--(xmax,ymin)*u--(xmax,ymax)*u--(xmin,ymax)*u--cycle;
axe_x = (xmin,0)*u -- (xmax,0)*u;
axe_y = (0,ymin)*u -- (0,ymax)*u;
drawarrow axe_x;
drawarrow axe_y;
draw pts_f withpen pencircle scaled 1;
clip currentpicture to screen;
\stopMPcode
\stoptext

Are there any routines with Metafun to improve this code?
thank you,
Fabrice