What instructions did you follow for installing ipympl?

Just pip install ipympl.

It works now after running the following bash commands.

conda install -y nodejs
pip install --upgrade jupyterlab
jupyter labextension install @jupyter-widgets/jupyterlab-manager
jupyter labextension install jupyter-matplotlib
jupyter nbextension enable --py widgetsnbextension

Edit: Must restart jupyter server after running these commands

Thanks, "restart" was the critical instruction for me. It wasn't enough the quit and start again jupyterlab in the terminal, only after restarting the computer the plots finally worked

Usually clearing the browser cache is the important step

FWIW I had same problem, but had to avoid using conda, and was required to use virtual envs with python.
I was able to fix this by following nodejs's install instructions using a package manager. In my case this was ubuntu -- so using nodejs binary distributions from github.

This ended up being something like (in total):

mkvirtualenv -p python3.8 jupenv
pip install jupyter matplotlib pandas ipympl tqdm
curl -sL https://deb.nodesource.com/setup_10.x | sudo -E bash -
sudo apt-get install -y nodejs
jupyter labextension install @jupyter-widgets/jupyterlab-manager

Because of the venv, you later start jupyter with:

workon jupenv
jupyter lab

Hope this helps others.

Same problem here, restarting kernel not work for me. only when I restart jupyter lab from terminal it works. (%matplotlib widget)

upgrade pip
add

pip install ipympl 

in anaconda prompt
and restart jupyter notebook