Support window in the transform array

[kanitw]

Given the new joinaggregate transform, we should also support this in the transform array.

One possible syntax is to have a join: boolean flag in the SummarizeTransform.

[kanitw]

Since @domoritz agree with the join: boolean flag, @AkshatSh you can directly modify AggregateNode and the type definition for SummarizeTransform.

• I also notice that SummarizeTransform doesn't support cross and drop in Vega's aggregate transform yet, so probably worth adding them too.
  • Note that cross and drop are in aggregate but not in joinaggregate so we will probably have to make some minor type adjustment.

[kanitw]

• Right now AggregateNode always output type: 'aggregate', but with join: true -- we wanna do joinaggregate.

[kanitw]

@AkshatSh Let's not add the join flag for now as Jeff is working on a new window transform and will deprecate joinaggregate. That said, supporting cross and drop in aggregate could still be useful :)

[kanitw]

Note that now a window transform has landed. You may want to add a window transform to VL. :)

[kanitw]

I think window transform syntax should be pretty similar to Vega, but should not have type (type: window) just like our summarize transform does not have type: 'aggregate'.

Please propose a syntax and we can discuss :)

[kanitw]

Good cases to demonstrate in examples:

• Percent of total
• Different from total/mean
• Percent difference from total/mean

[kanitw]

Another example:

• rank chart as in https://stats.stackexchange.com/a/178344

[kanitw]

Will be fixed in #3199