How to offset an axis label

labelOffset moves the axis label in the direction orthogonal to the axis (say horizontally for the y axis). But how would you move it in the direction of the axis?

Suppose you want to move the axis label down (because there is a scale.insetTop or a large marginTop).

A hacky solution (implemented in https://observablehq.com/@recifs/a-centered-title) is an explicit label with lots of \n (spaces if working on the x axis)

Plot.axisY({ label: "\n\n\n\n\n↑ flipper_length_mm" }),

Similarly, a text mark with facetAnchor+frameAnchor.

Note: dy doesn't apply to the axis label (this is probably a bug in itself? this was intentional); if this worked, we could create a specific axis just for the label, and offset it. It would be tedious, but would benefit from the automatic label.

I think we should:

• fix the bug with dy (#1517)
• introduce a new option orthogonal to labelOffset (name?) (#1516)

The issue does not name files or tests. Start by reviewing the axis-label API and the related work referenced in #1516 and #1517; done means axis labels can be positioned along the axis without newline or spacing hacks.