\documentclass{standalone}
\usepackage{luamplib}
\begin{document}
\begin{mplibcode}
beginfig(1);
 numeric w, h, n; w = 377; h = 80; n = 500;

 draw (origin--right) scaled w;
 draw (origin--right) scaled w shifted (0,+h/2) withcolor 3/4;
 draw (origin--right) scaled w shifted (0,-h/2) withcolor 3/4;

 pair zenith, nadir; zenith = nadir = origin;
 color gain, lose; gain = .67 blue; lose = .85 red;
 for i=1 upto 8:
   path walk;
   numeric y; y = 0;
   walk = origin for x=w/n step w/n until w:
     hide(
       y := y if uniformdeviate 1 < 1/2: + else: - fi 1;
       if y > ypart zenith: zenith := (x,y) ; fi
       if y < ypart nadir:  nadir  := (x,y) ; fi
     )
     -- (x,y)
   endfor;
   undraw walk withpen pencircle scaled 3/4;
   draw walk withcolor (1/2+y/h)[lose, gain];
 endfor
 draw origin withpen pencircle scaled 2;
 drawarrow (12 up -- 2 up) shifted zenith withcolor gain;
 drawarrow (12 down -- 2 down) shifted nadir  withcolor lose;
endfig;
\end{mplibcode}
\end{document}