Success! I used the correct repository BUT it turned out that for some unknown reason the directory \lib\chart.js\dist was listed in .gitignore. This created a big confusion, since the directory was on my local workstation but not in the actual Github repository, which I failed to notice. That also explains why the app worked with ionic serve but not once compiled with Phonegap. It works great now, thank you for your hints!
Have a nice weekend.