I've just recently had a discussion with a coauthor while I was preparing an article for submission. For comparison between two effects, I had wished to use an image (a simple line graph) from another article (after getting the required permission), but my coauthor seemed to disagree. They said that we should simply use a digitizer to sample the points of the graph, as the image has copyright, but the data exists within the public domain once published, then we replot and publish with a citation.
This seems odd to me, and raises the question, to what end could this work? I see this making sense if we are constructing a plot with multiple components, but here we would just be reconstructing a plot nearly verbatim. Further, if we had an automated digitizer and ploted the retreived data with similar axes, this method would approach a simple copy and paste, which obviously requires copyright permissions.
So, in general, is this a true statement, and valid opinion, or would copyright still be required?