Support xOffset and yOffset as encoding channels

[tuner]

Current situation

xOffset and yOffset are supported as mark properties and they translate the viewport. In addition, dx and dy are are sometimes supported as channels ("point" mark), sometimes as mark properties ("text" mark), and sometimes not at all. This is incompatible with Vega-Lite, which supports dx and dy as a mark properties in its "text" mark but not elsewhere. And nowhere as an encoding channel.

What to do

It might make sense to support xOffset and yOffset as channels instead and also implement x2Offset and y2Offset. In addition, dx and dy would be mark properties solely in "text" mark. However, the gene annotation track relies on dx as a channel to adjust the strand triangles' placement. Thus, this would be a breaking change.

Use cases

Lollipops

The y2Offset property would be useful with "lollipop" plots, where the bottom of the "stick" should be moved outside the scale to make a contact with the protein:

(source: https://www.mdpi.com/1422-0067/20/14/3442)

SV paws

The paws of the SV arcs can be implemented using the "point" mark. However, controlling the paw's "stroke width" is impossible and getting it's size to align nicely with the arc's stroke width is tricky. An alternative approach would be to use the "rule" mark and map the strand (paw direction) to xOffset with a nominal scale and a range comprising a negative and a positive value. Thus, xOffset would shift x to either left or right side of the arc's endpoint. In this case, x2Offset should be set to zero.