If this MapVsGeo has already been posted on another subreddit you can additionally specify its commit-ID here:
Some data can be automatically retrieved via the Reddit API. This step is optional but can be quite handy at times.
If there's also an "original" post on another subreddit:
This assistant can try to automatically request the coordinates using Nominatim.
Add this snippet to data/ext/markers.json.js (alphabetically sorted by the commentId) and create a commit with the description .
data/ext/markers.json.js