prologues := 3;
outputtemplate := "%j/%j-%c.svg";
outputformat := "svg";

verbatimtex
%&latex
\documentclass{article}
\usepackage[charter]{mathdesign}
\usepackage{amsmath}
\begin{document}
etex

u:=1.2cm;
path para;
picture env[];

def milieu(expr a,b) = .5[a,b] enddef;

for i:=(-200) upto (200):
beginfig(i+200);
  pair P,M,O,N,Q;
  path drPQ, medPQ, horiP,seg;
  seg:=(-0.1u,-0.1u)--(.1u,.1u);
  O:=(0,0);
  Q:=(2u,0);
  drawarrow (-u,0)--(7u,0);
  drawarrow (0,-4u)--(0,4u);
  P:=(0,i/50*u);
  N:= milieu(P,Q);
  drPQ:=12[P,Q]--12[Q,P];
  medPQ:= drPQ rotatedaround (N,90);
  horiP:= (-12u,ypart P)--(12u,ypart P);
  M:= horiP intersectionpoint medPQ;
  env[i]:=image(%
    draw medPQ withcolor 0.9*white;
    );
  if i=-200:
    para:=M;
  else:
    para:=para--M;
  fi;
  for j:=-200 step 8 until i:
    draw env[j];
  endfor;
  draw drPQ withcolor green dashed evenly withpen pencircle scaled 0.8pt;
  draw medPQ withcolor green dashed evenly withpen pencircle scaled 0.8pt;
  draw horiP withcolor green dashed evenly withpen pencircle scaled 0.8pt;
  draw para withcolor red withpen pencircle scaled 1pt;
  draw seg shifted milieu(Q,N);
  draw seg shifted milieu(P,N);

  dotlabel.llft(btex $O$ etex,O);
  dotlabel.urt(btex $Q$ etex,Q);
  dotlabel.lft(btex $P$ etex,P);
  dotlabel.llft(btex $N$ etex,N);
  dotlabel.urt(btex $M$ etex,M);
  label.llft(btex $x$ etex,(7u,0));
  label.lrt(btex $y$ etex,(0,4u));
  label(btex \textit{La parabole} etex, (5u,2u));
  clip currentpicture to (-u,-4u)--(7u,-4u)--(7u,4u)--(-u,4u)--cycle;
endfig;
endfor;
end.