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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def plot_intrusion_curve(self, fig=None):
 r""" Plot the percolation curve as the invader volume or number fraction vs the applied capillary pressure. """

        # Begin creating nicely formatted plot
        x, y = self.get_intrusion_data()
        if fig is None:
            fig = plt.figure()
        plt.semilogx(x, y, 'ko-')
        plt.ylabel('Invading Phase Saturation')
        plt.xlabel('Capillary Pressure')
        plt.grid(True)
        return fig