Custom HTML in new tab (plugin)

I would like to create a new plugin or modify OctoPrint UI itself. I want to add this new tab:

The data will come from the measure device connected to Arduino. The data (in some kind of txt/json) would be stored in root of OctoPrint directories. This will manage another Python script - that is not problem. My struggle is how to create plugin, that can read that txt file and show it in the tab of OctoPring UI.
Thank you for any help!

Edit: In fact I just need to add new tab that could (for example) carry one main div and javascript code - this code will load data from txt file & render it. HTML/JS is ok, but I don't know anything about plugins.

Have you checked the documentation, in particular the plugin tutorial yet?

Yes, I did. But I don't know where to start. And I don't want to waste time with whole documentation, you know.. Thank you for reply

Start at the plugin tutorial, and follow that to learn how plugins work & how you can make one.

Then you're not really looking to make a plugin, you just are looking for one that does what you want already made. The Hello World example in the tutorial ends up getting you most the way there for what you want (a tab that loads a page in an iframe). You would just have to save your text file in html format and configure haproxy on the pi to serve that page to link to. (actually looks like haproxy will load the page into memory and not reload until service restart, so this bit wouldn't work). You would just have to define a custom http route hook in your plugin to serve the file.