Improve Crossfilter/ResolveFilter API about metis HOT 1 CLOSED

What I don't like about this approach is

1. removing a crossfilter means figuring out where these references are and removing them.
2. it adds magic string control flow to the graph when I thought the whole point of the graph was to represent control flow structurally.

But if that's how VEGA does it, I know they've thought a lot about this problem…

My suggestion is this:
Given that user

1. crossfilters chart A, then

     src
     / \
    /   xf1 = {transforms: [{type: "filter", expr: "recipient_party = 'D'"}]}
   /   /   \
chA  chB     chC

2. crossfilters (xf2) chart C, then

        src
        / \
       /    xf1
    xf2     / \ = {transforms: [{type: "filter", expr: "recipient_state = 'CA'"}]}
   /   \   /   \
chA     chB     chC

3. uncrossfilters A:

     src
     / \
    /   xf1
   /   /   \
chA  chB     chC

So what happens if we evaluate Chart B while it has two parents 😱 ?
We concat the expressions

{transforms: [
  {type: "filter", expr: "recipient_party = 'D'"},
  {type: "filter", expr: "recipient_state = 'CA'"}
]}

from metis.