On the suggestion of Sven from the Gimp project I have added offset display to the features of pnginfo. It’s bundled in the 0.3 release.
A historical note from November 2020: this code is quite old, but still actively used. I have therefore converted the old subversion repository to git and it is hosted at https://github.com/mikalstill/pngtools. I will monitor there for issues and patches and try my best to remember what I was thinking 20 years ago…