code: def delta(var, key, tusec=None):
      '''calculate slope'''
      global last_delta
      if tusec is not None:
          tnow = tusec * 1.0e-6
      else:
          import mavutil
          tnow = mavutil.mavfile_global.timestamp
      dv = 0
      ret = 0
      if key in last_delta:
          (last_v, last_t, last_ret) = last_delta[key]
          if last_t == tnow:
              return last_ret
          if tnow == last_t:
              ret = 0
          else:
              ret = (var - last_v) / (tnow - last_t)
      last_delta[key] = (var, tnow, ret)
      return ref delta(var, key, tusec=None):
      '''calculate slope'''
      global last_delta
      if tusec is not None:
          tnow = tusec * 1.0e-6
      else:
          import mavutil
          tnow = mavutil.mavfile_global.timestamp
      dv = 0
      ret = 0
      if key in last_delta:
          (last_v, last_t, last_ret) = last_delta[key]
          if last_t == tnow:
              return last_ret
          if tnow == last_t:
              ret = 0
          else:
              ret = (var - last_v) / (tnow - last_t)
      last_delta[key] = (var, tnow, ret)
      return ret
docs: calculate slope