Immediate quick fix patch to get you up and running:
You can change the source code for the plugin directly to change the link to https, Which the line you want to change is line 9 of assets/javascripts/lib/discourse-markdown/discourse-yuml.js.es6github link.
Of course this means it isn’t really configurable, but it’ll get you up and running.
Longer term
On top of that, If you are pretty comfortable with writing discourse plugins, you can add a configuration option yourself, that alters the url and submit a pull request. That’ll always be muchly appreciated.
Though, If you aren’t comfortable doing that, you can open an issue in the github project, and I’ll get round to making it configurable option once I’ve got some spare time.
Sorry, I’ve only just spotted this. Latest update now allows you to have multiple diagrams per post. It was something I spotted myself, and fixed it a few weeks ago.
You changed the code but the link didn’t change to https? Have you tried force refreshing the page. I don’t think the plugin code would get cached by the browser, but it might be.
When did this change? as I’m running an up to date discourse, though not over https (as it’s an internal network) and the plugin still works for me.
In that case, just remove the plugin and reinstall it, if you can log an issue on the github project, and I’ll get round to adding a HTTPs option (or auto detect which would be best) when I get some time.