Support stacked position option in line layers about giraffe HOT 2 CLOSED

I did some investigation.

• We can either build the stacking transform ourselves, or use d3
• d3 outputs data in a row-oriented format that will be a pain to integrate with our current PlotEnv / Table interfaces
• There's a lot of fantastic parameters to a stacked line graph that encourage different visualization insights. I would love to expose some of these parameters in the line layer config

from giraffe.

Building our own stacking transform is the easiest way to go.

Separately, other challenges to consider:

• shading each layer with its own color rather than the mixed color of the stacked layers
• yScale and yDomain need to be updated appropriately
• line hover tooltip has a separate column for the cumulative value at each layer

from giraffe.