code: func linear(x1, y1, x2, y2 float64) (slope, intercept float64) {
  	if x2 != x1 {
  		slope = (y2 - y1) / (x2 - x1)
  		intercept = y2 - x2*slope
  	}
  	return
  }
docs: // linear returns the slope and y-intercept of the straight line joining the
  // two specified points. For scaling purposes, associate the arguments as
  // follows: x1: observed low value, y1: desired low value, x2: observed high
  // value, y2: desired high value.