'Adds an element from a plot\'s series to an existing plot.
Examples
Consider two ``Plot`` objects, ``p1`` and ``p2``. To add the
second plot\'s first series object to the first, use the
``append`` method, like so:
>>> from sympy import symbols
>>> from sympy.plotting import plot
>>> x = symbols(\'x\')
>>> p1 = plot(x*x)
>>> p2 = plot(x)
>>> p1.append(p2[0])
>>> p1
Plot object containing:
[0]: cartesian line: x**2 for x over (-10.0, 10.0)
[1]: cartesian line: x for x over (-10.0, 10.0)
See Also
extend'

@doctest_depends_on(modules=('numpy', 'matplotlib'))
def append(self, arg):

    if isinstance(arg, BaseSeries):
        self._series.append(arg)
    else:
        raise TypeError('Must    specify    element    of    plot    to    append.')
