from which paper is this plot? May be we have the datapoints.

Cheers, Rainer

You can use 

http://arohatgi.info/WebPlotDigitizer/app/

to extract data from graph

There are many- The following pieces of software are also good for this application:

1) Origin

2) ScanIt: https://www.amsterchem.com/scanit.html

3) DataThief: https://www.datathief.org/

4) Plotdigitizer: http://plotdigitizer.sourceforge.net/

5) Graphdigitizer: http://www.silkscientific.com/graph-digitizer.htm

6) Getdata- Graph-digitizer: http://getdata-graph-digitizer.com/

I hope this helps

Webplotdigitizer is the best and free. It works online as a well as offline

https://youtu.be/PW1nYO8ayMc