Plum allows you to toggle a "plumb-line" guide in the glyph view that remains centered between the side bearings of the glyph.
https://github.com/jackjennings/Plum
Plum requires RoboFont 1.2 or greater.
Either install Plum through Mechanic, or download the .zip file and double click Plum.roboFontExt.
- Toggle the center line with a key command
- Keeps the plumb-line centered