Extend existing data via REST

Hello guys,
I’ve uploaded a csv and ploted a graph.
I saved it, shared a link to it.
I saw on some page a link to ‘add new data via REST’ but I closed the window and since I’ve been in incognito mode I don’t have a history to check.
So I’m asking you guys,
What is the way to extend the file I uploaded via REST?
I don’t use python, R, etc. Just plain REST.