docs: def plot_voltage(network, boundaries=[]):
      """
      Plot voltage at buses as hexbin
  
  
      Parameters
      ----------
      network : PyPSA network container
      boundaries: list of 2 values, setting the lower and upper bound of colorbar
  
      Returns
      -------
      Plot
      """
  
      x = np.array(network.buses['x'])
      y = np.array(network.buses['y'])
  
      alpha = np.array(network.buses_t.v_mag_pu.loc[network.snapshots[0]])
  
      fig, ax = plt.subplots(1, 1)
      fig.set_size_inches(6, 4)
      cmap = plt.cm.jet
      if not boundaries:
          plt.hexbin(x, y, C=alpha, cmap=cmap, gridsize=100)
          cb = plt.colorbar()
      elif boundaries:
          v = np.linspace(boundaries[0], boundaries[1], 101)
          norm = matplotlib.colors.BoundaryNorm(v, cmap.N)
          plt.hexbin(x, y, C=alpha, cmap=cmap, gridsize=100, norm=norm)
          cb = plt.colorbar(boundaries=v, ticks=v[0:101:10], norm=norm)
          cb.set_clim(vmin=boundaries[0], vmax=boundaries[1])
      cb.set_label('Voltage Magnitude per unit of v_nom')
  
      network.plot(
          ax=ax, line_widths=pd.Series(0.5, network.lines.index), bus_sizes=0)
      plt.show()