Draw a Marey graph in SVG for a pattern (collection of trips in a route
    that visit the same sequence of stops).

def handle_GET_ttablegraph(self,params):
    """Draw a Marey graph in SVG for a pattern (collection of trips in a route
    that visit the same sequence of stops)."""
    schedule = self.server.schedule
    marey = MareyGraph()
    trip = schedule.GetTrip(params.get('trip', None))
    route = schedule.GetRoute(trip.route_id)
    height = int(params.get('height', 300))

    if not route:
      print('no such route')
      self.send_error(404)
      return

    pattern_id_trip_dict = route.GetPatternIdTripDict()
    pattern_id = trip.pattern_id
    if pattern_id not in pattern_id_trip_dict:
      print('no pattern %s found in %s' % (pattern_id, pattern_id_trip_dict.keys()))
      self.send_error(404)
      return
    triplist = pattern_id_trip_dict[pattern_id]

    pattern_start_time = min((t.GetStartTime() for t in triplist))
    pattern_end_time = max((t.GetEndTime() for t in triplist))

    marey.SetSpan(pattern_start_time,pattern_end_time)
    marey.Draw(triplist[0].GetPattern(), triplist, height)

    content = marey.Draw()

    self.send_response(200)
    self.send_header('Content-Type', 'image/svg+xml')
    self.send_header('Content-Length', str(len(content)))
    self.end_headers()
    self.wfile.write(content)