Grouped marks (area and line) filter differently on different channels

[mbostock]

The area and line marks have special filtering behavior for their positional channels (x and y for line; x1, y1, x2, and y2 for area). If some of the x values are NaN, for example, you’ll get gaps:

Here x is defined as:

d => d.unemployment < 14 ? d.date : NaN

However, if I do something similar with the stroke channel, the line will interpolate across missing values:

Here stroke is defined as:

d => d.unemployment < 14 ? d.division : NaN

The latter seems like a bug: it would be preferable to show gaps if any of the values are missing for required channels rather than interpolating over the missing values. This is related to the desire to specify custom reducers for grouped values, rather than being limited to just showing the first value for each group. #724