Cars MPG example plot (#496)

* This example plot computes the median of cars' economy (mpg), grouped by number of cylinders

The bins are sorted by decreasing r, so that they are all visible.

The example would benefit from stackR (#197).

It could also benefit from a strategy to create missing values for the line, so that it's broken when there are no data. However, it won't work with an approach such as "return empty bins" (#495), because returning empty bins will not create the *z* values for each and every category, which would be necessary if we wanted to create broken lines. This shows that a generic foolproof solution to #351 will require much more than #495 (and #489 and #491 are not better in that regard).

* Update test/plots/cars-mpg.js

Co-authored-by: Mike Bostock <[email protected]>

* Update test/plots/cars-mpg.js

Co-authored-by: Mike Bostock <[email protected]>

* zero, not filter

* group, not bin

* remove console.log

* stroke, not fill

Co-authored-by: Mike Bostock <[email protected]>

Author: Fil