As long as we all are still waiting for these large-scale data sharing mechanisms in place we will have to use every bit we can can our hand on. For data that is only represented as a graph - instead of tables or in the text - this means that we need some kind of graph digitizer.
The best one I found is the webplotdigitizer developed by Ankit Rohatgi. While I generally prefer desktop versions of programs, because they support reproducible research - this tools is just too easy to use that a further search would be necessary. Together with the video tutorials this is a one-stop shop for your data-extraction needs.
Congratulations to the developer for this excellent example of one task one tool!