// draw through (xt,yt) with endpoints (x0,y0) and (x2,y2)
  x1 = xt * 2 - (x0 + x2)/2; 
  y1 = xt * 2 - (y0 + y2)/2; 
  moveTo(x0,y0);
  curveTo(x1,y1,x2,y2);