I'm working on a manuscript for a scientific paper that alters/adds onto what another study found over 20 years ago. This old study is highly cited and the graphs are reproduced in a lot of textbooks, but my study says that the old data isn't the whole story. In part of my manuscript I compare my data to that in the old study and I'd like to include a graphical comparison. How do I contact the authors of the old study and ask for permission to reproduce their graph? Do I tell them about my study? Do I let them read my manuscript?
It's appropriate to contact the authors, particularly if they are still active researchers. Before sending them the manuscript or telling the entire story, perhaps ask if they would be willing to keep your results confidential until publication.
As far as formal permission to reproduce their graph, it's very likely that the authors don't hold the copyright. Most authors sign over copyright to the publisher, so you may have to query them. Even so, it would be polite and advisable to notify the authors that you are planning to reproduce a piece of their work for comparison.