I agree, unit tests wouldn't be very useful here, i was thing that visual regression tests with constant data, that capture graph image and compare with older version for regressions would have been really nice here.
As a side note: I find these kind of tests flaky, because the antialiasing doesn't seem stable at constant resolution on the same machine & browsers when you repeat tests. When you do a fuzzy compare (ie. allow for X pixels difference), then you are always chasing the golden middle. A test I would like to see for a control is trigger multiple state transitions (for example change 5 properties of a chart), do assertions on the generated DOM.
Unless maybe some HNer knows how to do such SVG/picture testing non flaky?
