If you want to digitize a plot in an article or report to e.g. do some calculations with them, there is a good program, which is free and cross platform: Plot Digitizer. You scan a plot to png or gif, or cut out a plot from a pdf file and save it as png and gif. Load it into Plot Digitizer, tell it where the minimal and maximal x- and y-values are – voila: you “cracked” the plot.