docs: 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