def show_vertex(self, node_id, show_ce_ratio=True):
        """Plot the vicinity of a node and its ce_ratio.

        :param node_id: Node ID of the node to be shown.
        :type node_id: int

        :param show_ce_ratio: If true, shows the ce_ratio of the node, too.
        :type show_ce_ratio: bool, optional
        """
        # Importing matplotlib takes a while, so don't do that at the header.
        from matplotlib import pyplot as plt

        fig = plt.figure()
        ax = fig.gca()
        plt.axis("equal")

        # Find the edges that contain the vertex
        edge_gids = numpy.where((self.edges["nodes"] == node_id).any(axis=1))[0]
        # ... and plot them
        for node_ids in self.edges["nodes"][edge_gids]:
            x = self.node_coords[node_ids]
            ax.plot(x[:, 0], x[:, 1], "k")

        # Highlight ce_ratios.
        if show_ce_ratio:
            if self.cell_circumcenters is None:
                X = self.node_coords[self.cells["nodes"]]
                self.cell_circumcenters = self.compute_triangle_circumcenters(
                    X, self.ei_dot_ei, self.ei_dot_ej
                )

            # Find the cells that contain the vertex
            cell_ids = numpy.where((self.cells["nodes"] == node_id).any(axis=1))[0]

            for cell_id in cell_ids:
                for edge_gid in self.cells["edges"][cell_id]:
                    if node_id not in self.edges["nodes"][edge_gid]:
                        continue
                    node_ids = self.edges["nodes"][edge_gid]
                    edge_midpoint = 0.5 * (
                        self.node_coords[node_ids[0]] + self.node_coords[node_ids[1]]
                    )
                    p = _column_stack(self.cell_circumcenters[cell_id], edge_midpoint)
                    q = numpy.column_stack(
                        [
                            self.cell_circumcenters[cell_id],
                            edge_midpoint,
                            self.node_coords[node_id],
                        ]
                    )
                    ax.fill(q[0], q[1], color="0.5")
                    ax.plot(p[0], p[1], color="0.7")
        return
Plot the vicinity of a node and its ce_ratio.

        :param node_id: Node ID of the node to be shown.
        :type node_id: int

        :param show_ce_ratio: If true, shows the ce_ratio of the node, too.
        :type show_ce_ratio: bool, optional