WebPlotDigitizer (http://arohatgi.info/WebPlotDigitizer/ moved to https://automeris.io/WebPlotDigitizer/) is a free online, browser-based, open-source tool developed by Ankit Rohatgito to extract data from a wide variety of graphs and figures. Well worth checking out! There is also a downloadable version and R package which I haven’t tried but could be useful for others.
Of course, extracting data from published figures could be a little dodgy depending on how the data is used, but having said that, if you need to re-graph data, because of lack of access to the raw data, for teaching or whatever purpose, this is a very handy and easy to use tool.
I recently wanted to extract the data from the graphs of several old papers. One thing was to roughly compare the absorption of wavelengths of light by the different chlorophylls in plants. I couldn’t easily find data to graph but there are many spectra published, such as in this excellent textbook Plants In Action. Using WebPlotDigitizer I could easily extract the data from the spectra here and redraw them quickly in Excel as below.
(redrawn from figure 1.8, Plants in Action, Edition 2, ASPA originally based on J.R. Evans and J.M. Anderson, BBA 892: 75-82, 1987)