'Returns a string containing a new-line separated
list of the functions in the function list.'

def __str__(self):

    s = ''
    if (len(self._functions) == 0):
        s += '<blank    plot>'
    else:
        self._render_lock.acquire()
        s += '\n'.join([('%s[%i]:    %s' % ('', i, str(self._functions[i]))) for i in self._functions])
        self._render_lock.release()
    return s
