def split_float_colons(string_values):
    """
    Uses numpy-ish syntax to parse a set of two floats.  Blanks are interpreted as None.

    Parse the following:
       1:10
       1:
       :100
       1.1:200
       1:300.
    """
    if string_values:
        assert ',' not in string_values, string_values
        sline = string_values.split(':')
        assert len(sline) <= 2, sline
        values = []
        for svalue in sline:
            if svalue == '':
                val = None
            else:
                val = float(svalue)
            values.append(val)
    else:
        assert string_values is None, None
        values = None
    return values
