Support Wilkinson Dot Plots

[kanitw]

This will allow us to nicely create the Wilkinson Dot Plots.

A few superb examples that inspire me:

http://graphics.wsj.com/job-market-tracker/

http://roadtolarissa.com/nba-minutes/

To do this, we need to:

• allow stacking point
• allow putting _id in the field (esp. for detail)
• allow using mid point of the stack operation instead of stack start / end

[kanitw]

ID can be done using row_order operator in the pending window transform #2488.

[kanitw]

Note that currently non-ranged mark (e.g. point, line) uses stack end point for it position (to allow overlaying point and line on top of stacked area. However, when stacking point along like in Wilkinson dot plot. We will need to use the mid position.

Two possible solutions:

1. Use stack transform and manually calculate mid point
2. Extend the stack property to support new type of "zero" offset. (Maybe call it zero-mid?)

[kanitw]

Closing as this should be done as a part of #3819