How to accurately digitize plot data from figures available as vector graphics?
This is an amusing programming/hacking challenge, but my guess is that in 90% of cases the best solution lies in the realm of human affairs, and that’s simply
to email the authors and ask for the data.
Why? Two reasons:

When it works (and I expect it would most of the time), you’ll know with certainty that the data you have is exactly what the authors were working with rather than some approximation recovered by trying to reverse engineer an unknown sequence of human and algorithmic processes to convert the raw data into a figure. (Vector graphics may offer the illusion of lossless encoding, but that’s assuming no lossy steps were applied by the human or software at any step along the way — a dangerous assumption to make in practice.)

In the infrequent occasion when it doesn’t work because the authors refuse to share the data, you’ll still learn something useful about how trustworthy their data can be assumed to be (i.e., not at all). You can still try to revert to a technological solution, but in most cases I’d just assume the data is invalid and not worth relying on.