The bridge is controlled using the Stem library, which communicates with the Tor process over the Tor control protocol. The setup is managed in the BeagleBridge class. After establishing a connection with the Tor process, this class registers two event listeners for the Bandwidth and Configuration changed event. The bandwidth event is triggered each second and reports, via the print_bw callback, the bytes used in the last second. This information is used to draw the bandwidth graph. The following callback function shows how the callback interacts with the LCD:

def make_bw_callback(test,lcd):
  '''Returns a callback function for the bandwidth event'''
  def print_bw(event):
    '''Obtains the bandwidth used from the last second from the
       bridge, normalizes it to the total bandwidth, and draw 
       that information to the display'''
    up = int(test.get_up_ratio(event.written))
    down = int(test.get_down_ratio(event.read))
    lcd.display_graph...