Software for extracting data from a graph without having to click on every single point? There are many times when I am faced with the task of extracting data from a published graph (usually a bitmap image in an paper). For example, a scatter plot from which I would like to get a list of individual (x, y) coordinates for the points.

One option is to ask the contact author for raw data. Most will do it, sometimes in nice ASCII format, sometimes in Excel files, sometimes in formats that I cannot open (chemists are fond of software like Origin or Igor Pro). Some authors never reply, or ask questions like “what do you want to do with it?”. In all cases, it takes time. Sometime, it's not even possible (I can hardly email the author of a 1936 paper!).

The other option is to extract the data. I currently use g3data to do that, but for large scatter plots having to click on every single point is tedious. Thus, I am looking for a data extraction software that could recognize individual points automagically, and possibly filter them by point color or symbol used. Is that even something that exists? What other tools can you recommend to work around this issue?

I don't think it'd be appropriate to have extra requirements on the software, so I'm happy with free or commercial solutions, running on any OS. Of course, if given the choice, I'd prefer open source software running on Linux and Mac OS.
Here is a very good online tool: http://arohatgi.info/WebPlotDigitizer/app/