How can I add a background circle to a sunburst plot?

How do I add a background circle to a sunburst plot in d3? I'm using the standard zoomable sunburst example:

enter image description here

I just want a full circle of specified color, to highlight the 'jaggy-ness' of a particular display.

Solution

All you need to do is append a circle to the SVG as the first thing after creating it:

svg.append("circle").attr("r", radius).style("fill", "pink");

Complete example here.