It is often necessary to obtain data values from graphs, e.g. from scanned scientific plots, laboratory logs and others. You can either eyeball the values of interest or use a digitizing tool when there are many data points or precision is required.
PlotDigitizer offers an online application that is free to use but has limited features. However, the free online version is adequate for basic tasks and is really easy to use.
First you upload the graph image to PlotDigitizer and select the graph type (2D Scatter, Bar, Pie, Polar and Ternary graphs are available in the free version). In my case, I needed the CPT tip resistance (qc) values from Zhang et al. (2002). Next, you calibrate the axis/axes (Linear, Logarithmic, Date/Time available in the free version) by selecting their endpoints (note X1, X2, Y1 and Y2) and typing their values (0, 10, 0 and 20 respectively). Then you start marking the data points by clicking the data points on the graph, which automatically generates their values. A very helpful feature is the zoom window on the top right.
Finally, you can copy to the clipboard or export these data to other formats (csv, xls and others).
The pro license (payment required) offers additional features that can really boost productivity and save time, like auto-tracing and dataset distinction. So, one could consider purchasing the pro license if there are many graphs to digitize or is often in the need to digitize.
Check out a quick review video down the video gallery section.