Generates a 2D plot for the z=0 Polygon projection.
        
        :param color: Polygon color.
        :type color: matplotlib color
        :param alpha: Opacity.
        :type alpha: float
        :param ret: If True, returns the figure. It can be used to add 
            more elements to the plot or to modify it.
        :type ret: bool
        :returns: None, axes
        :rtype: None, matplotlib axes

def plot2d(self, color='default', alpha=1, ret=True):
        """
        Generates a 2D plot for the z=0 Polygon projection.
        
        :param color: Polygon color.
        :type color: matplotlib color
        :param alpha: Opacity.
        :type alpha: float
        :param ret: If True, returns the figure. It can be used to add 
            more elements to the plot or to modify it.
        :type ret: bool
        :returns: None, axes
        :rtype: None, matplotlib axes
        """
        import matplotlib.pyplot as plt
        import matplotlib.patches as patches
        
        path = self.get_path()
        domain = self.get_domain()[:, :2]

        if color is 'default': color = 'b'

        # Plot
        fig = plt.figure()
        ax = fig.add_subplot(111)
        ax.add_patch(patches.PathPatch(path, facecolor=color, lw=1, 
                                       edgecolor='k', alpha=alpha))
        ax.set_xlim(domain[0,0],domain[1,0])
        ax.set_ylim(domain[0,1], domain[1,1])
        if ret: return ax