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.
I personally feel the the complexity added to the test by checking the entire path is not worth being able to check each point.  The unit tests should be as pinpoint as possible.