<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def plot(self):
 """ plot the activation function """

    plt.ion()
    plt.show()
    x = numpy.linspace(0, 15, 100)
    y = numpy.zeros(x.shape)
    y = self.excite(y, x)
    plt.plot(x, y)
    plt.xlabel('Input')
    plt.ylabel('Persistence')
    plt.title('Sigmoid Activation Function')