Matplotlib.pyplot not working on Mac in virtualenv

I had a very curious problem where python would not find pyplot in matplotlib.

I was using:

• python3
• virtualenv
• matplotlib

To create the virtual environment I used: 

virtualenv -p python3 <myproject name>

However, when I used:

python3 -m venv <myproject name>

It worked ok.

An explanation was found here:
https://matplotlib.org/faq/osx_framework.html