I’m trying to make an interactive plot that has a regression line on top of it. Here is the plot in the Vega editor and here is a gist of the Altair source (MRE). This is heavily influenced by this very nice SO example (thanks Andrii Syd).

In summary, I want to plot a bunch of points (and be able to toggle them on/off by clicking on the legend) and also a regression line on top of them (and be able to toggle that on/off as well by clicking on the legend). I would prefer to have it toggle in this way rather than what I see as less elegant user inputs (checkboxes, etc.) that are placed beneath the chart. I have both conceptual and practical questions.

1. I simply don’t understand the apparently necessary use of the transform_fold call. It works, it’s necessary, and I just don’t get what it’s supposed to be doing and why it’s necessary. The SO post is great in solving OP’s problem. It just doesn’t explain why.
2. I have a base plot (of points) and then the regression plot on top of it. I don’t get, conceptually, how I use selection_point to have similar behavior with both points and line. I have used it to control opacity of both points and lines, and added it to both layers, and it does what I expect for points but not for the line (the line won’t toggle on/off). I’m trying to understand where I should add selection_point (and other similar calls like interactive) when I have multiple layers (on each layer individually? on one layer only? on the result when they are all layered on top of each other?).
3. The points and line are mixed together (alphabetically) in the legend; it’d be nicer to have all the points up top under a Series heading, then the line next under a Fit heading.
4. It would also be nice to have the line with a different “icon” (say, a line, not a circle).