Pseudo-infinite cycle

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.**

The issue names no files, tests, or entry points. First locate the existing cycle API and its tests, then determine how a pseudo-infinite cycle should represent initial colors followed by reuse of the final color. Done means the behavior is defined, implemented, and covered for multiple curves.

Written by the indexing model from the issue text.