I don’t like the use of a hard-coded interval (1) here because it won’t behave well if the scale domain changes. It’d also be nice if we could achieve the minor/major grid lines using a single grid mark; this example currently draws major lines atop minor lines, which isn’t always an acceptable solution.

it's possible with a variable strokeOpacity, that depends on the tick (d) or on the tick rank (i), but it has to be refined by hand, since it's not easy to know (generically) which tick is major or minor.

For the sake of moving forward quickly, I’m going to drop the minor grid lines from this example, but leave the use of interval (1).