Account for NaN and undefined values on Plot.Line and Plot.Area

Makes use of d3's `defined()` option on its `line` and `area` generators to skip `NaN` and `undefined` values.

Adjusted `Plot.Line` and `Plot.Area` unit tests to create a new `Plot` for each test, reducing cross-dependency between tests.

Quicktests coming shortly.
The second argument has to be applied to satisfy the signature of `IAppliedAccessor`. `.defined()` has the wrong signature in d3.d.ts, so the compiler yells at me if I try to have the index as a parameter in the anonymous function ... I'll fix this....