Horizon Chart d3v4

testing a d3v4 version of horizon plugin

Work progressing on Github at d3-horizon. Follow along or even better help out!

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**forked from mbostock's block: Horizon Chart**

Horizon charts combine position and color to reduce vertical space. Start with a standard area chart, then mirror negative values (in blue) or offset them vertically. Click the + button above to increase the number of bands, turning the area into a horizon.

Implemented with the d3.horizon plugin.