Return a string containing the range of values.

def _get_str_range(self, vals_stats):
        """Return a string containing the range of values."""
        minmax = vals_stats[1] # stats minmax
        minval = self.fmtstr.format(minmax[0])
        maxval = self.fmtstr.format(minmax[1])
        return '{A} to {B:6>}'.format(A=minval, B=maxval)