[Pharo MOOC] Object/Class Diagram generation

Hey, all.

I was just wondering if anyone knew how the Pharo MOOC was creating their Object/Class Diagrams like so:

<img src="file:///Users/sergio/Library/Group Containers/2E337YPCZY.airmail/Library/Application Support/it.bloop.airmail2/Airmail/General/Local/1546531486991106048/Attachments/E0BA4E69-4369-45C7-A7E6-846301984FB1.png">