'Draw the sentence string.'

def _draw_sentence(self):

    if (self._chart.num_leaves() == 0):
        return
    c = self._sentence_canvas
    margin = ChartView._MARGIN
    y = ChartView._MARGIN
    for (i, leaf) in enumerate(self._chart.leaves()):
        x1 = ((i * self._unitsize) + margin)
        x2 = (x1 + self._unitsize)
        x = ((x1 + x2) / 2)
        tag = c.create_text(x, y, text=repr(leaf), font=self._font, anchor='n', justify='left')
        bbox = c.bbox(tag)
        rt = c.create_rectangle((x1 + 2), (bbox[1] - (ChartView._LEAF_SPACING / 2)), (x2 - 2), (bbox[3] + (ChartView._LEAF_SPACING / 2)), fill='#f0f0f0', outline='#f0f0f0')
        c.tag_lower(rt)
