storeAngle = function(d) {
                this._current = d;
            }
Reusable Donut Chart API class that renders a
simple and configurable donut chart.

@module Donut
@tutorial donut
@requires d3-dispatch, d3-ease, d3-interpolate, d3-scale, d3-shape, d3-selection

@example
var donutChart = donut();

donutChart
    .externalRadius(500)
    .internalRadius(200);

d3Selection.select('.css-selector')
    .datum(dataset)
    .call(donutChart);