Unsetting the register manually an error will be raised:

    >>> pd.set_option("plotting.matplotlib.register_converters",
    ...               False)  # doctest: +SKIP
    >>> df.plot.line(x='ts', y='y')  # doctest: +SKIP
    Traceback (most recent call last):
    TypeError: float() argument must be a string or a real number, not 'Period'
    r