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.

More details at "https://markummitchell.github.io/engauge-digitizer/"