Engauge Digitizer

You can also use im2graph to digitize graphs (convert from images to numbers).

im2graph is a digitizing software that converts graph-images to graph-data, that is to numbers. With behind the scenes fast and efficient image processing algorithms, im2graph effortlessly converts images to graphs with a click of a button. For more complex use cases, a highly tuned user-assisted GUI is available, enabling fast and effortless conversions from graph images to values.

im2graph is free and available on Windows and Linux.

See http://www.im2graph.co.il

• somewhat automatic, segments lines but you need to choose which segments are the plot
• needs to remove gridlines then can fit a curve
• handles rotation, scaling and skew, but not warp
• segmentation is based on hard thresholds of pixels, so it produces jaggy, piecewise linear output instead of smooth curves. like it should really be combining information from all the pixels in the cross-section of the plot to find the true point position, but it doesn't.
• has plain zoom, but no magnifier?
• can save and reopen progress