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.
This path string honestly says little to me for the test.  Can we check for an existence of the line splitting?  For example, I see 2 Z's, which I think means that there are 2 line endings.  We should use this logic to pinpoint what's being tested.