experiment with vega formula transform and new zip functionality

Vega Experiments with Stats and Formula Transform and New Zip

Using the barley.json example from the vega package, I wanted to try to add plots with x as

1. Yield Difference From Site Mean
2. Yield Difference From Aggregate Mean

Please do not replicate, since I'm guessing I'm performing this ugly hack instead of the more elegant preferred method that I have overlooked. New projects are like puzzles, and I'm still a long way from completing this one.