\def{text F=\parm1}
\def{integer n=\parm2}
\def{integer nn=\parm3}
\def{text f=item(1,\F)}
\def{text g=item(2,\F)}
\def{text a=item(3,\F)}
\def{text b=item(4,\F)}
\def{real tt=\a}
\def{real xxv=evalue(\f,t=\tt)}
\def{real yyv=evalue(\g,t=\tt)}
\def{text dessin=linewidth 3
line \minx-1,\ysize1+0.5,\maxx+1,\ysize1+0.5, green
linewidth 1
dline \minx-1,\ysize1+0.5,\xxv,\yyv,green
disk \minx-1,\ysize1+0.5,6, green
disk \xxv,\yyv,6, green
}
\for{i=1 to \n}
{\def{real tt=\a+\i*(\b-(\a))/(\n)}
\def{real ttfaux=\minx-1+\i*(\maxx-(\minx)+2)/(\n)}
	\def{real xx=evalue(\f,t=\tt)}
	\def{real yy=evalue(\g,t=\tt)}
	\def{text dessin=\dessin
linewidth 1
	dline \ttfaux,\ysize1+0.5,\xx,\yy,green
linewidth 1
	line  \xx,\yy,\xxv,\yyv,red
	disk \xx,\yy,6, green
	disk \ttfaux,\ysize1+0.5,6, green
	}
	\def{ text xxv=\xx}
	\def{ text yyv=\yy}
}
\draw{200,200*\prop}
	{
	xrange \xsize1,\xsize2
	yrange \ysize1,\ysize2
	hline  0,0, grey
	vline 0,0, grey
	trange \a,\b
	linewidth 2
	plot navy, \f,\g
	\dessin
	}