'The configure callback.  This is called whenever the window is
resized.  It is also called when the window is first mapped.
It figures out the unit size, and redraws the contents of each
canvas.'

def _configure(self, e):

    N = self._chart.num_leaves()
    self._unitsize = ((e.width - (2 * ChartView._MARGIN)) / N)
    self.draw()
