docs: def plot_sens_center(self, frequency=2):
          """
          plot sensitivity center distribution for all configurations in
          config.dat.  The centers of mass are colored by the data given in
          volt_file.
          """
          try:
              colors = np.loadtxt(self.volt_file, skiprows=1)
          except IOError:
              print('IOError opening {0}'.format(volt_file))
              exit()
  
          # check for 1-dimensionality
          if(len(colors.shape) > 1):
              print('Artificial or Multi frequency data')
              colors = colors[:, frequency].flatten()
  
          colors = colors[~np.isnan(colors)]
  
          elem.load_elem_file(self.elem_file)
          elem.load_elec_file(self.elec_file)
          nr_elements = len(elem.element_type_list[0])
          elem.element_data = np.zeros((nr_elements, 1)) * np.nan
  
          elem.plt_opt.title = ' '
          elem.plt_opt.reverse = True
          elem.plt_opt.cbmin = -1
          elem.plt_opt.cbmax = 1
          elem.plt_opt.cblabel = self.cblabel
          elem.plt_opt.xlabel = 'x (m)'
          elem.plt_opt.ylabel = 'z (m)'
  
          fig = plt.figure(figsize=(5, 7))
          ax = fig.add_subplot(111)
          ax, pm, cb = elem.plot_element_data_to_ax(0, ax, scale='linear',
                                                    no_cb=True)
          ax.scatter(self.sens_centers[:, 0], self.sens_centers[:, 1], c=colors,
                     s=100, edgecolors='none')
  
          cb_pos = mpl_get_cb_bound_next_to_plot(ax)
          ax1 = fig.add_axes(cb_pos, frame_on=True)
          cmap = mpl.cm.jet_r
          norm = mpl.colors.Normalize(vmin=np.nanmin(colors),
                                      vmax=np.nanmax(colors))
          mpl.colorbar.ColorbarBase(ax1, cmap=cmap, norm=norm,
                                    orientation='vertical')
  
          fig.savefig(self.output_file, bbox_inches='tight', dpi=300)