> What is the return value of boxplotContainer.append("line")? How do I debug it?

I don’t know D3, but usually in these chaining approaches it would be something like:

    boxplotContainer
    .append("line")
    .debug(callable_thing)
    .attr(…)

Where debug() might be pipe() or some variation that lets you provide a callable where you can inspect the data (or drop into an interactive REPL, or whatever).