Plot the curve and the knee, if it exists

def plot_knee(self, ):
        """
        Plot the curve and the knee, if it exists
        
        """
        import matplotlib.pyplot as plt

        plt.figure(figsize=(8, 8))
        plt.plot(self.x, self.y)
        plt.vlines(self.knee, plt.ylim()[0], plt.ylim()[1])