曲线
曲率，curvature, 是一个衡量几何体弯曲程度的量。
edge中会经常涉及到曲率
curve-style, 'The curving method used to separate two or more edges between two nodes'.
有两个curve method比较常用：
- "curve-style: bezier", curved edges automatically spaced(自动).
- "curve-style": unbundled-bezier, complete manual control(手动).
对于控制曲线需要以下属性:
- control-point-step-size: For the line perpendicular from source to target, this value speciifyes the distance between successive bezier edges.
- control-point-distance : Overrides control-point-step-size with a manual value. Because it overrides the step size, bezier edges with the same value will overlap. Thus, it's best to use this as a one-off value for particular edges if need be.
- control-point-weight : Weights control points along the line from source to target. This value ranges on [0, 1], with 0 towards the source node and 1 towards the target node.
