def __call__(self, x, pos=None):
        """
        Return label for tick at coordinate x. Relative position of
        ticks can be specified with pos. First tick gets chromosome name.
        """
        s = self._format_val(x, prec_offset=1)
        if pos == 0 or x == 0:
            return "{}:{}".format(self.chromosome, s)
        return s