code: func (f *Fpdf) dounderline(x, y float64, txt string) string {
  	up := float64(f.currentFont.Up)
  	ut := float64(f.currentFont.Ut)
  	w := f.GetStringWidth(txt) + f.ws*float64(blankCount(txt))
  	return sprintf("%.2f %.2f %.2f %.2f re f", x*f.k,
  		(f.h-(y-up/1000*f.fontSize))*f.k, w*f.k, -ut/1000*f.fontSizePt)
  }
docs: // Underline text