You just have the wrong order of your commands. A backend should be set before importing pyplot in jupyter. Or in other words, after changing the backend, pyplot needs to be imported again.
Therefore call %matplotlib ...
prior to importing pyplot.
In first cell:
%matplotlib inline
import matplotlib.pyplot as plt
plt.plot([1,1.6,3])
In second cell:
%matplotlib notebook
#calling it a second time may prevent some graphics errors
%matplotlib notebook
import matplotlib.pyplot as plt
plt.plot([1,1.6,3])