code: func (g *Graph) Float64(def ...float64) float64 {
  	n, ok := _float64f(g.String())
  	if !ok {
  		if len(def) == 0 {
  			return math.NaN()
  		}
  		return def[0]
  	}
  	return n
  }
docs: // Float64 returns the node as a float64. If the node is not a number, it
  // return NaN, or the default value if given.