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 considered that; there's also some value in checking that the remaining points were drawn in the right pixel locations, which is why I went with this test.