Stack improvements

[Fil]

negative values; offsets; sort by z, stacked lines, transforms

• Negative values always result in diverging stacking. (By contrast, in d3-shape an explicit diverging offset must be invoked.)
• Offsets: expand and silhouette are similar to d3-shape, but taking into account negative values.
• Grouping can be defined by z, fill, stroke…
• z and an optional zOrder define the ordering of the stack
• stacked lines with position=<top (default),center,bottom> or <right (default),center, left>
• transform: ƒ

closes #109
closes #22
closes #113

examples:

s:

[Fil]

I think we can merge this. In the future we might want to add:

• sorting (a sort option that could be reverse, or a function that receives and sorts the z, or a sort method similar to d3's; with only z everything is possible, since we can pre-sort in a specific field and call z on that field)
• more offset options (free-form function, wiggle)

[Fil]

Converted this PR to Draft since I want to reconsider a few things.

In brief:

• rename x y channels as key, value (trivial)
• z is for grouping (make it more explicit)
• order groups with stackOrder strategies (todo)
• use an accessor for the value, so we don't have to populate with 0 values to have curves that gradually "emerge"
• …

[mbostock]

Exciting to see so much progress!

[Fil]

rebased and squashed as #132