I've created an experimental/beta/dev version of the app which can be downloaded here:
https://1drv.ms/u/s!Ak5mj_EVpj-P0joyW9urp2ILFw1_
The dev version should allow for the ability to rerecord a "partial line" for a side_l.csv or side_r.csv file, but the CSV must first be recorded with this version (this version adds a PositionOnTrack column to the CSV, which AC ignores, but the app can then read later). If you try to "load" a CSV saved with a previous version of this app it will not work (and may crash AC).
The "Track Left" and "Track Right" buttons work the same and will require you to trace the line for a full lap (same as before). The "Part Left" and "Part Right" buttons will attempt to load the previously saved CSV file (must have been saved with this dev version or higher), and they also require you to drive a full lap, but you don't have to "trace" the whole time. This means you can quickly drive to the spot you want to change, trace the new section, the quickly drive to the S/F.
When you click the "Part Left" button it loads the saved CSV into a "dictionary", or "dic". When you pass S/F it will use the values from the dic until you click the F4 hotkey (I'd map this to a button on my wheel using JoyToKey) or the "Part Left" button, which toggles on live data. It will continue using the live data until you click F4 or the button again to toggle back to the dic. When it is pulling data from the dic the title will read "dicLeft", and when it is pulling live data it will read "trackLeft". Hopefully that all makes sense, as I'm not at a place where I can record a demo video.
I've been able to do basic testing with this, so I think it should work, but I'm not currently able to do extensive testing, so I'm putting it out here as a "beta" version for anyone that would like to try the new features and give me feeback.
The PoT has been expanded to 6 decimal places, and the app will not record more than 1 world position for each PoT (in case you were wondering).
If you have any issues using this dev version please let me know.
Cheers,
Esotic