'Write txt at pos in canvas with specified font
and color.
Return text item and x-coord of right bottom corner
of text\'s bounding box.'

def _write(self, pos, txt, align, font, pencolor):

    (x, y) = pos
    x = (x * self.xscale)
    y = (y * self.yscale)
    anchor = {'left': 'sw', 'center': 's', 'right': 'se'}
    item = self.cv.create_text((x - 1), (- y), text=txt, anchor=anchor[align], fill=pencolor, font=font)
    (x0, y0, x1, y1) = self.cv.bbox(item)
    self.cv.update()
    return (item, (x1 - 1))
