I have something figured out but have not got the complete picture though.
That error is raised by np.gradient function when the array passed to that function does not have enough elements to compute gradient. This is mainly caused by the skip_start and skip_end arguments in the function recorder.plot().
Cheap and dirty way would be to do learner.recorder.plot(skip_start=0, skip_end=0) then all the learning rates will be considered. Otherwise, the default arguments trim the learning rates.