%matplotlib notebook showing a blank histogram

The answer is not necessarily to restart the entire kernel.

If you reload the matplotlib module, it will work, too. Provided you use Python 3.6 like me, and you have import matplotlib.pyplot as plt like me:

from importlib import reload
reload(plt)
%matplotlib notebook

It does the trick. Yes it is still a hack. At least this is an independent codecell you can use in the middle of the notebook. Switching back via %matplotlib inline is not a problem.

You can also remove once imported names from the sys.modules list, then they get imported again when you call the import again.

import sys
sys.modules.pop('matplotlib')
from matplotlib import pyplot as plt

In many cases, that's a less good idea. But it might sometimes be the only straw to hold on.

I was able to fix it by downgrading matplotlib to 3.1.3:

conda install matplotlib=3.1.3

I was running version 3.3.2 and had this same issue. I was not switching between %matplotlib inline and %matplotlib notebook, and it did not matter if I placed %matplotlib notebook before or after importing

Seeing that my comment above has indeed helped someone to solve the problem I will post it as an answer.

The problem occurs if you switch from %matplotlib inline to %matplotlib notebook without restarting the kernel.

Switching from %matplotlib notebook to %matplotlib inline works fine.

So the solution is to either restart the kernel or start a new notebook.

It seems that in some cases it helps to repeat the setting of the notebook backend, i.e. call it twice like

%matplotlib notebook
%matplotlib notebook

An analysis for why that is can be found in this comment