Matlab

>> getdata;

* open
* Xmin Xma
  Ymin Yma
* Iniation
  move the cursor to Xmin, Xma, Ymin, Yma and click them respectively in order
  Press "Enter"
* Getdata
  move the cursor and click the points you want on the curve
  Press "Enter"
* save

Note that the "txt" file can only be saved once. Restart "getdata"
and follow the same precedure if someting goes wrong.

$ engauge &

* import "JPG" figure
* set three points coordinate
  origin
  upper left
  upper right
* manually choose data points of the curve
* export file as "csv" file

http://digitizer.sourceforge.net/usermanual/tutormanpointgraph.html