code: def _calculate_pen_up_strokes(self, pointlist, times=None):
          
          if times is None:
              times = []
          for i in range(len(pointlist) - 1):
              stroke_info = {"start": pointlist[i][-1]['time'],
                             "end": pointlist[i + 1][0]['time'],
                             "pen_down": False}
              x, y, t = [], [], []
              for point in [pointlist[i][-1], pointlist[i + 1][0]]:
                  if point['time'] not in t:
                      x.append(point['x'])
                      y.append(point['y'])
                      t.append(point['time'])
              if len(x) == 1:
                  # constant interpolation
                  fx, fy = lambda x: float(x), lambda y: float(y)
              else:
                  # linear interpolation
                  x, y = numpy.array(x), numpy.array(y)
                  fx = interp1d(t, x, kind='linear')
                  fy = interp1d(t, y, kind='linear')
              stroke_info['fx'] = fx
              stroke_info['fy'] = fy
              times.append(stroke_info)
          return times
docs: 'Pen-up' strokes are virtual strokes that were not drawn. It
              models the time when the user moved from one stroke to the next.