code: func linearTickmark(tm []float64, lo, hi float64) (slope, intercept float64) {
  	ln := len(tm)
  	if ln > 0 {
  		slope, intercept = linear(tm[0], lo, tm[ln-1], hi)
  	}
  	return
  }
docs: // linearTickmark returns the slope and intercept that will linearly map data
  // values (the range of which is specified by the tickmark slice tm) to page
  // values (the range of which is specified by lo and hi).