Add spacer

digraph G
{
  // Add space using width and height attributes.
  //  Recognized units are in, px, pc, pt, cm and mm 
  //  for inches, pixels, picas, points, centimeters and millimeters, respectively. 
  //  The default unit is points.
  c[label="Empty spacer" shape="none", width="3px" height="6px"];
  d[label="Unhidden Empty spacer" shape="box", width="3px" height="6px"];
  a->b;
  b->c;
  b->d;
}