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:
DigitGraph2.zip
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.
Output from the WebPlot program (both manual and automated) is shown below plotted together with output from my Digitgraph function, showing close agreement for all three lines.
Newton Excel Bach, not (just) an Excel Blog 
great contribution. Thanks
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Another option is Enguage Digitizer, http://markummitchell.github.io/engauge-digitizer/. I’ve used it off and on for many years.
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Thanks Paul, I’ll have a look.
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Wow. I’m going to try and see if this works for the PCA slab-on-grade method charts. I wonder if it will handle multi-line graphs.
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I haven’t tried multi-line graphs, but the auto line detection uses the line colour, so if all the lines are the same colour I imagine it might have problems.
But the manual method would have no problem at all, other than being a bit tedious.
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