"UserWarning: Matplotlib is currently using agg, which is a non-GUI backend, so cannot show the figure." when plotting figure with pyplot on Pycharm

I found a solution to my problem (thanks to the help of ImportanceOfBeingErnest).

All I had to do was to install tkinter through the Linux bash terminal using the following command:

sudo apt-get install python3-tk

instead of installing it with pip or directly in the virtual environment in Pycharm.

In my case, the error message was implying that I was working in a headless console. So plt.show() could not work. What worked was calling plt.savefig:

import matplotlib.pyplot as plt

plt.plot([1,2,3], [5,7,4])
plt.savefig("mygraph.png")

I found the answer on a github repository.

If you use Arch Linux (distributions like Manjaro or Antegros) simply type:

sudo pacman -S tk

And all will work perfectly!