I want to say thank you to Jeffery Grossman who took the time to import code and translate it.
Direct Download to English Version: https://github.com/jmgrossma/freeftys/blob/master/secondary/NppHorizontalRuler/NppHorizontalRuler.dll?raw=true
Drop this .dll into the C:\Program Files (x86)\Notepad++\plugins\ folder and restart NPP. Note that the x64 version won’t even have a plugin manager. See below for more info on that.
I’ve tested it and confirmed it works.
This took me a little bit of digging to find but after a lot of searching – this ruler plugin came up in my search. The page is in Japanese, so run it through google translate or use the download link here:
Direct download link: http://freesftys.lv9.org/app/npp/soft/NppHorizontalRuler-0.1.6.zip
Extract and install NppHorizontalRuler.dll here:
Note: This plugin will not work with Npp x64 – The official website specifically says “most of plugins (including Plugin Manager) are not yet available in x64“.
Windows 7: C:\Program Files (x86)\Notepad++\plugins\
Once installed, it will look like this:
(Update Feb 27 2017: English Translation thanks to Franco in the comments)