It’s from maybeDenseInterval. It’s the same idea as line and area. And it’s why LinearRegressionOptions extends BinOptions. For example for areaY you can specify an interval along x, and then reduce defaults to first to extract the first value in y for the current interval in x.

Edit: Though, perhaps our approach here could use some improvements. Like, perhaps this should be more of a select transform than a bin transform? Because I don’t think this is likely to work for channels that don’t end up getting binned by maybeDenseInterval. It might only appear to work today because we’ve only tested it on simple instantiations?

I've documented what works. Can you open an issue with what doesn't? (It's not clear to me what's missing here.)

these represent the dependent variable, and should probably go into their respective mark (x in linearRegressionX, and y in linearRegressionY)

done