Filter + groupby breaks if filter includes target data

[rreusser]

The example below works if target is omitted but breaks if target is filled in, even when it's exactly the same as the x data that does work. Each transform works independently but the combination fails only when target data is specified.

I'm assuming this means groupby is not outputting the correctly grouped target data. Perhaps findDataAttributes doesn't know how to recurse into other transforms.

https://codepen.io/rsreusser/pen/vZjxWM?editors=0010

[rreusser]

A small note: findDataAttributes does know how to recurse into other transforms. Instead it was failing to actually no-array-clone the transform and then split data arrays into grouped subsets. The result was that they all overwrote each other, it got confused, and the result had no data. That's been fixed in #1892.