def append_data(self, t, sp, vel, error):
    self.time[self._iter_num] = t
    self.setpoint[self._iter_num] = sp
    self.actual_velocity[self._iter_num] = vel
    self.error[self._iter_num] = error
    self._iter_num += 1
def append_data(self, t, sp, vel, error):
    self.time[self._iter_num] = t
    self.setpoint[self._iter_num] = sp
    self.actual_velocity[self._iter_num] = vel
    self.error[self._iter_num] = error
    self._iter_num += 1