Radar plot basic

Referencing issue #982 
Should cover the first 5 points of that issue.

Note: branch targeted to radarPlot as a feature branch
Note: only one dataset can be added and while a dataset can have multiple datums, a single datum is the only tested behavior on my end.
in other words.... this._metrics = this._metrics.concat(metrics).

why does this take splat arguments rather than a list? its inconsistent with api for other things like scale.domain or scale.range