code: func (s *Surface) PointAt(x, y int) *Point {
  	if x >= s.Width || y >= s.Height {
  		panic("geo: x, y outside of grid range")
  	}
  
  	p := NewPoint(0, 0)
  
  	p[0] = s.bound.sw.X() + float64(x)*s.gridBoxWidth()
  	p[1] = s.bound.sw.Y() + float64(y)*s.gridBoxHeight()
  
  	return p
  }
docs: // PointAt returns the point, in the bound, corresponding to
  // this grid coordinate. x in [0, s.Width()-1], y in [0, s.Height()-1]