The clickloc.tk micro-app just opens an image file and prints out the pixel coordinates of any mouse clicks upon it. I use it to get rough numbers from journal figures. You can use scale click.py to convert the output to units of your choice, if you use your first four clicks to mark out the coordinate system (xmin,anything), (xmax,anything), (anything,ymin), and (anything,ymax).

$ pdfimages article.pdf fig
$ clickloc.tk fig-000.ppm > fig-000.pixels
$ scale_click.py 0 10 5 20 fig-000.pixels > fig-000.data

Take a look at plotpick for grabbing points from raw datafiles (which is more accurate and easier than reverse engineering images).