code: func (dc *Context) DrawStringAnchored(s string, x, y, ax, ay float64) {
  	w, h := dc.MeasureString(s)
  	x -= ax * w
  	y += ay * h
  	if dc.mask == nil {
  		dc.drawString(dc.im, s, x, y)
  	} else {
  		im := image.NewRGBA(image.Rect(0, 0, dc.width, dc.height))
  		dc.drawString(im, s, x, y)
  		draw.DrawMask(dc.im, dc.im.Bounds(), im, image.ZP, dc.mask, image.ZP, draw.Over)
  	}
  }
docs: // DrawStringAnchored draws the specified text at the specified anchor point.
  // The anchor point is x - w * ax, y - h * ay, where w, h is the size of the
  // text. Use ax=0.5, ay=0.5 to center the text at the specified point.