'Return the format for tick value `x` at position pos.
The position is ignored and the value is rounded to the nearest
integer, which is used to look up the label.'

def __call__(self, x, pos=None):

    i = int((x + 0.5))
    if ((i < 0) or (i >= self.n)):
        return u''
    else:
        return self.labels[i]
