This guide shows you how to add separators to plank.

Open the terminal and copy/paste the following;

sudo apt install git
git clone git://github.com/bipolarizm/PlankZer.git ~/PlankZer
cd ~/PlankZer
chmod +x plankzer.sh && ./plankzer.sh

If you want more than one separator, run the plankzer.sh script once more.

Enjoy! ^_^

Tested on: elementary OS 5.1.6