Support stacked position option in line layers

[chnn]

Similar to the position option in a histogram layer config, we should support a position option in line layer configs.

type LinePosition = 'overlaid' | 'stacked'

It should default to 'overlaid'.

The stacking logic should take place in a stat. We may have to refactor the collectLineData and simplifyLineData utils in the line layer to make this all work out.

[chnn]

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

[TCL735]

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