I have posted here previously a spreadsheet that allows XY data to be extracted from images of graphs, maps or other images of objects in a single plane: How to digitise a scanned image.
I have updated the instructions for the procedure to cover Excel’s new dynamic array feature, which can return an array of data from a function entered in a single cell. The new version can be downloaded from:
I also recently discovered the Webplot Digitizer program, that can either be used on-line, or as a download. The download is free (with no advertising), with a button for voluntary donations. The screen-shot below shows the downloaded version. The program is considerably more sophisticated than my spreadsheet, providing not only a magnified image of the graph at the cursor location, but also allowing the option of either manual selection of data points, or a fully automated process to detect the graph line(s) and generate the data points.