Pseudo-infinite cycle

还没有人认领这个 Issue。

It could be useful to have the possibility to define a pseudo-infinite cycle which gives some first colors and then use the last one defined in the cycle.

Why this could be useful ? When you have several curves, it could be better to just colorize some curves to highlight them whereas the others are less important but you still need to show them in gray for example.

**Here is [an example](http://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1003833) : see the rule 8.**

该 issue 没有指明文件、测试或入口点。首先找到现有的 cycle API 及其测试，然后确定伪无限 cycle 应如何表示初始颜色，随后复用最终颜色。完成的标准是：该行为已定义、已实现，并且已针对多条曲线覆盖测试。

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