I really hate seeing this used in discussion. Just because it's open source does not mean it's immune from criticism. Yes, technically some random HN'er could spend tens of hours writing unit tests and submit a PR, but really this is a task for the library author/maintainer.
Having tests is a measure of quality, and should be cared about if you want people using your library.
We need to stop thinking of OSS as something we're entitled to, that if a stranger doesn't put in enough free labour we're allowed to complain about it. I try to view my dependencies as favours other people have done for me. You wouldn't nitpick about a lack of tests after someone wrote a whole charting library for you; you'd thank them and add your own tests.
My point was: Many are dogmatic about tests these days, and I find it refreshing there are no tests.
Tests can be added later for something like this library, if/when the need arises - or when someone decides to spend "tens of hours" writing them.
Tests can be a measure of quality, but they aren't the only measure - and do not apply in all cases