Escuse for my english...
I wanted to use this plugin in my 5.8 WordPress installation, but it didn't work because of changing of Drawio.it web service to embed draw diagrams, and the error visualized reference a new url for embedding new service: "embed.diagrams.net"
Were 4 years that the plugin wasn't updated, so i decided to experiment something...
... tried to substitute in the installed plugin, in all files where present, only the string: "www.draw.io" in to the string: "embed.diagrams.net"
I had to change only two files: drawit.js, drawit.php!!
Then I tried again to use DrawIt to draw a diagram, and everything worked well, I could edit a complex schema in a page, with arrows, colours, rotations, and then I could modify it and finally publish correctly the result.
I really wish the owner of the plugin update the plugin in this way (or better), because is very usefull for many persons.