For text, allow x and y to be specified as constant numbers

[mbostock]

For manual labels, it’d be nice if you could specify x and y as constant numbers. Currently they can either be a channel or null; in the latter case the labels are placed in the center of the chart.

Related, it might also be nice to have an anchor option which allows the text to be positioned somewhere else relative to the surrounding frame; this would eliminate the need to hard-code dx and dy (e.g., in the geofacet example). The anchor option could accept values such as top, top-left, top-right, right, etc.

[Fil]

Related #153

[mbostock]

Related multi-line text #677 #313 #327. Specifically #677 introduces a lineAnchor property which controls vertical position of lines relative to the anchor point.

[mbostock]

I don’t think we want to introduce this convention of literal positioning, and especially that we’d want x and y to behave differently for text marks. But I could see a frameAnchor option that controls the position for otherwise position-less marks; it would default to middle for the current behavior.

[mbostock]

Closing in favor of #683.