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'm looking at the old version and I see
d3.svg.area()
    .x(stuff)
    .y(stuff)
    .y0(stuff)

Why cant we append a
    .defined(stuff)

to the end?