https://github.com/d3/d3/blob/master/API.md#chords-d3-chord

chord生成器将两段弧形连接起来

d3.chord - create a new chord layout.
chord - compute the layout for the given matrix.
chord.padAngle - set the padding between adjacent groups.
chord.sortGroups - define the group order.
chord.sortSubgroups - define the source and target order within groups.
chord.sortChords - define the chord order across groups.
d3.ribbon - create a ribbon shape generator.
ribbon - generate a ribbon shape.
ribbon.source - set the source accessor.
ribbon.target - set the target accessor.
ribbon.radius - set the ribbon source or target radius.
ribbon.startAngle - set the ribbon source or target start angle.
ribbon.endAngle - set the ribbon source or target end angle.
ribbon.context - set the render context.