How do researchers use published plots?
I know of four approaches, and to my knowledge, all are commonly used. I'm coming from theoretical chemistry, so not quite theoretical physics, but not so different.


Often the data is made public, especially from large collaborations. For example: http://opendata.atlas.cern/ Otherwise, ask the original authors for their data. Often they'll share.
Recalculate the results -- especially the case with theoretical/computational approaches. Within a given field, many methods are usually quite similar to each other, so code written to do one can be easily modified to handle another. It just costs you some computer time.
Have a standard dataset that you compare to. If you're developing new methods, you test them against a standard that has been studied deeply. I've especially seen this in computational work, where a single massive brute-force simulation is used for years as a benchmark, and that data is shared with the community.
Graph digitizers, e.g., http://markummitchell.github.io/engauge-digitizer/. Scan and estimate where the data is. (Actually, this is the only approach I personally have not really used.)