arc.innerRadius()

arc.innerRadius([radius])

If radius is specified, sets the inner radius to the specified function or number and returns this arc generator. If radius is not specified, returns the current inner radius accessor, which defaults to:

function innerRadius(d) {
  return d.innerRadius;
}

Specifying the inner radius as a function is useful for constructing a stacked polar bar chart, often in conjunction with a sqrt scale. More commonly, a constant inner radius is used for a donut or pie chart. If the outer radius is smaller than the inner radius, the inner and outer radii are swapped. A negative value is treated as zero.