The Engauge Digitizer tool accepts image files (like PNG, JPEG and TIFF)
containing graphs, and recovers the data points from those graphs. The resulting
data points are usually used as input to other software applications.
Conceptually, Engauge Digitizer is the opposite of a graphing tool that converts
data points to graphs. The process is shown below - an image file is imported,
digitized within Engauge, and exported as a table of numeric data to a text
file. Work can be saved into an Engauge DIG file.

WWW: http://markummitchell.github.io/engauge-digitizer/