constant numeric x and y for text marks

[Fil]

position the text relative to the anchor

anchor can be any of "top", "bottom", "left", "right" or combinations such as "top-left"

demo: https://observablehq.com/@observablehq/text-facet-alignment-523

closes #523

todo:

• [x] create unit tests for the possible anchors
• [ ] should "right" and "bottom" be counted "inwards" like in css?
• [ ] generalize to all marks?

[Fil]

A question: here we add x and y regardless of the anchor, but we could decide that the sign of the "right" and "bottom" positions should be negated (to go "inset"). I don't know which is the most intuitive.

[Fil]

converted to draft with the two questions that came up in review

[Fil]

I've now generalized the feature to dots and images. Not sure if it can/should be done for other marks?

[Fil]

Demo https://observablehq.com/@fil/constant-numeric-x-y-594

[Fil]

Obsolete. This can be achieved with frameAnchor: left and dx: 10 instead of x: 10.