DrillDown in Custom Visual

OK, I solved the first part of the mystery. I started looking into this and I found that capabilities.json supports a top-level drilldown property. I experiemented by adding the drilldown property to a custom visual I am writing today.

Once I added the drilldown property and associated with a specific data role, I add two fields into the well for the myCategory role and I can see those familiar drilldown buttons showing up.

What's cool is that the drilldown buttons on the left work automatically. However, the button on the right does not work automatically. I suspect that the last part of the mystery is figuring out how the visuals API has you attach your code to an event handler so you can respond to the user drilling down on a D3 shape such as a bar or a bubble.