'Adds all series from another plot.
Examples
Consider two ``Plot`` objects, ``p1`` and ``p2``. To add the
second plot to the first, use the ``extend`` method, like so:
>>> from sympy import symbols
>>> from sympy.plotting import plot
>>> x = symbols(\'x\')
>>> p1 = plot(x*x)
>>> p2 = plot(x)
>>> p1.extend(p2)
>>> 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)'

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

    if isinstance(arg, Plot):
        self._series.extend(arg._series)
    elif is_sequence(arg):
        self._series.extend(arg)
    else:
        raise TypeError('Expecting    Plot    or    sequence    of    BaseSeries')
