From Joseph A. Huwaldt:
Plot Digitizer is an easy to use Java program that allows you to digitize data points off of scanned plots, scaled drawings, or orthographic photographs. Includes an automatic digitization feature that can automatically digitize many types of functional data. ability to semi-automatically digitize lines off a plot. The user simply indicates where the line is on the plot with a thick paint brush and the program attempts to automatically sort out the data from grid lines.