Didger is described as 'Advanced georeferencing, digitizing, coordinate conversion, and mapping features at an affordable price. This is the ultimate geoprocessing and data conversion tool for map makers, cartographers, geologist, oil and gas professional, or GIS analyst'. There are four alternatives to Didger for Linux, Online / Web-based, Windows and Mac. The best alternative is Engauge Digitizer, which is both free and Open Source. Other great apps like Didger are WebPlotDigitizer (Free, Open Source), Plot Digitizer (Free, Open Source) and PlotDigitizer (Freemium).
This open source, digitizing software converts an image file showing a graph or map, into numbers. The image file can come from a scanner, digital camera or screenshot. The numbers can be read on the screen, and written or copied to a spreadsheet.
A large quantity of published data is available only in the form of plots and it is often difficult to extract numerical data accurately out of these pictures. There are several softwares available to aid this process, but most are either paid or poorly written.
Plot Digitizer is a Java program used to digitize scanned plots of functional data. Often data is found presented in reports and references as functional X-Y type scatter or line plots. In order to use this data, it must somehow be digitized.