docs: def __onclick(self, event):
          """
          respond to mouse clicks in the plot.
          This function responds to clicks on the first (horizontal slice) plot and updates the vertical profile and
          slice plots
  
          Parameters
          ----------
          event: matplotlib.backend_bases.MouseEvent
              the click event object containing image coordinates
  
          """
          # only do something if the first plot has been clicked on
          if event.inaxes == self.ax1:
              
              # retrieve the click coordinates
              self.x_coord = event.xdata
              self.y_coord = event.ydata
              
              # redraw the cross-hair
              self.__reset_crosshair()
              
              x, y = self.__map2img(self.x_coord, self.y_coord)
              subset_vertical = self.__read_timeseries(x, y)
              
              # redraw/clear the vertical profile plot in case stacking is disabled
              if not self.checkbox.value:
                  self.__init_vertical_plot()
              
              # plot the vertical profile
              label = 'x: {0:03}; y: {1:03}'.format(x, y)
              self.ax2.plot(self.timestamps, subset_vertical, label=label)
              self.ax2_legend = self.ax2.legend(loc=0, prop={'size': 7}, markerscale=1)