[BUG]: Intervention charts look like crap

[jamiewaese-uncharted]

Our intervention charts look kinda bad, both on the workflow node and in the drilldown.

The problem is that the label start point is determined by the data value. For interventions that begin near the bottom of the chart, or interventions that have long names, this causes the label to spill out of the chart. Like this:

While we can't solve all the issues, we can make it a lot better with two modifications:

1. Make the label begin at the top of the chart. This may require putting an expression on the xOffset or yOffset property where you subtract the value of dy

and

2. Add a "limit" to the label text that is roughly equivalent to the height of the chart.

Sort of like this:

This page may be useful: vegalite - labels page